TOMOYO Linux Cross Reference
Linux/fs/dcache.c

   // SPDX-License-Identifier: GPL-2.0-only
   /*
    * fs/dcache.c
    *
    * Complete reimplementation
    * (C) 1997 Thomas Schoebel-Theuer,
    * with heavy changes by Linus Torvalds
    */
   
  /*
   * Notes on the allocation strategy:
   *
   * The dcache is a master of the icache - whenever a dcache entry
   * exists, the inode will always exist. "iput()" is done either when
   * the dcache entry is deleted or garbage collected.
   */
  
  #include <linux/ratelimit.h>
  #include <linux/string.h>
  #include <linux/mm.h>
  #include <linux/fs.h>
  #include <linux/fscrypt.h>
  #include <linux/fsnotify.h>
  #include <linux/slab.h>
  #include <linux/init.h>
  #include <linux/hash.h>
  #include <linux/cache.h>
  #include <linux/export.h>
  #include <linux/security.h>
  #include <linux/seqlock.h>
  #include <linux/memblock.h>
  #include <linux/bit_spinlock.h>
  #include <linux/rculist_bl.h>
  #include <linux/list_lru.h>
  #include "internal.h"
  #include "mount.h"
  
  #include <asm/runtime-const.h>
  
  /*
   * Usage:
   * dcache->d_inode->i_lock protects:
   *   - i_dentry, d_u.d_alias, d_inode of aliases
   * dcache_hash_bucket lock protects:
   *   - the dcache hash table
   * s_roots bl list spinlock protects:
   *   - the s_roots list (see __d_drop)
   * dentry->d_sb->s_dentry_lru_lock protects:
   *   - the dcache lru lists and counters
   * d_lock protects:
   *   - d_flags
   *   - d_name
   *   - d_lru
   *   - d_count
   *   - d_unhashed()
   *   - d_parent and d_chilren
   *   - childrens' d_sib and d_parent
   *   - d_u.d_alias, d_inode
   *
   * Ordering:
   * dentry->d_inode->i_lock
   *   dentry->d_lock
   *     dentry->d_sb->s_dentry_lru_lock
   *     dcache_hash_bucket lock
   *     s_roots lock
   *
   * If there is an ancestor relationship:
   * dentry->d_parent->...->d_parent->d_lock
   *   ...
   *     dentry->d_parent->d_lock
   *       dentry->d_lock
   *
   * If no ancestor relationship:
   * arbitrary, since it's serialized on rename_lock
   */
  int sysctl_vfs_cache_pressure __read_mostly = 100;
  EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
  
  __cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
  
  EXPORT_SYMBOL(rename_lock);
  
  static struct kmem_cache *dentry_cache __ro_after_init;
  
  const struct qstr empty_name = QSTR_INIT("", 0);
  EXPORT_SYMBOL(empty_name);
  const struct qstr slash_name = QSTR_INIT("/", 1);
  EXPORT_SYMBOL(slash_name);
  const struct qstr dotdot_name = QSTR_INIT("..", 2);
  EXPORT_SYMBOL(dotdot_name);
  
  /*
   * This is the single most critical data structure when it comes
   * to the dcache: the hashtable for lookups. Somebody should try
   * to make this good - I've just made it work.
   *
   * This hash-function tries to avoid losing too many bits of hash
   * information, yet avoid using a prime hash-size or similar.
   *
  * Marking the variables "used" ensures that the compiler doesn't
  * optimize them away completely on architectures with runtime
  * constant infrastructure, this allows debuggers to see their
  * values. But updating these values has no effect on those arches.
  */
 
 static unsigned int d_hash_shift __ro_after_init __used;
 
 static struct hlist_bl_head *dentry_hashtable __ro_after_init __used;
 
 static inline struct hlist_bl_head *d_hash(unsigned long hashlen)
 {
         return runtime_const_ptr(dentry_hashtable) +
                 runtime_const_shift_right_32(hashlen, d_hash_shift);
 }
 
 #define IN_LOOKUP_SHIFT 10
 static struct hlist_bl_head in_lookup_hashtable[1 << IN_LOOKUP_SHIFT];
 
 static inline struct hlist_bl_head *in_lookup_hash(const struct dentry *parent,
                                         unsigned int hash)
 {
         hash += (unsigned long) parent / L1_CACHE_BYTES;
         return in_lookup_hashtable + hash_32(hash, IN_LOOKUP_SHIFT);
 }
 
 struct dentry_stat_t {
         long nr_dentry;
         long nr_unused;
         long age_limit;         /* age in seconds */
         long want_pages;        /* pages requested by system */
         long nr_negative;       /* # of unused negative dentries */
         long dummy;             /* Reserved for future use */
 };
 
 static DEFINE_PER_CPU(long, nr_dentry);
 static DEFINE_PER_CPU(long, nr_dentry_unused);
 static DEFINE_PER_CPU(long, nr_dentry_negative);
 
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS)
 /* Statistics gathering. */
 static struct dentry_stat_t dentry_stat = {
         .age_limit = 45,
 };
 
 /*
  * Here we resort to our own counters instead of using generic per-cpu counters
  * for consistency with what the vfs inode code does. We are expected to harvest
  * better code and performance by having our own specialized counters.
  *
  * Please note that the loop is done over all possible CPUs, not over all online
  * CPUs. The reason for this is that we don't want to play games with CPUs going
  * on and off. If one of them goes off, we will just keep their counters.
  *
  * glommer: See cffbc8a for details, and if you ever intend to change this,
  * please update all vfs counters to match.
  */
 static long get_nr_dentry(void)
 {
         int i;
         long sum = 0;
         for_each_possible_cpu(i)
                 sum += per_cpu(nr_dentry, i);
         return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_unused(void)
 {
         int i;
         long sum = 0;
         for_each_possible_cpu(i)
                 sum += per_cpu(nr_dentry_unused, i);
         return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_negative(void)
 {
         int i;
         long sum = 0;
 
         for_each_possible_cpu(i)
                 sum += per_cpu(nr_dentry_negative, i);
         return sum < 0 ? 0 : sum;
 }
 
 static int proc_nr_dentry(const struct ctl_table *table, int write, void *buffer,
                           size_t *lenp, loff_t *ppos)
 {
         dentry_stat.nr_dentry = get_nr_dentry();
         dentry_stat.nr_unused = get_nr_dentry_unused();
         dentry_stat.nr_negative = get_nr_dentry_negative();
         return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
 }
 
 static struct ctl_table fs_dcache_sysctls[] = {
         {
                 .procname       = "dentry-state",
                 .data           = &dentry_stat,
                 .maxlen         = 6*sizeof(long),
                 .mode           = 0444,
                 .proc_handler   = proc_nr_dentry,
         },
 };
 
 static int __init init_fs_dcache_sysctls(void)
 {
         register_sysctl_init("fs", fs_dcache_sysctls);
         return 0;
 }
 fs_initcall(init_fs_dcache_sysctls);
 #endif
 
 /*
  * Compare 2 name strings, return 0 if they match, otherwise non-zero.
  * The strings are both count bytes long, and count is non-zero.
  */
 #ifdef CONFIG_DCACHE_WORD_ACCESS
 
 #include <asm/word-at-a-time.h>
 /*
  * NOTE! 'cs' and 'scount' come from a dentry, so it has a
  * aligned allocation for this particular component. We don't
  * strictly need the load_unaligned_zeropad() safety, but it
  * doesn't hurt either.
  *
  * In contrast, 'ct' and 'tcount' can be from a pathname, and do
  * need the careful unaligned handling.
  */
 static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
         unsigned long a,b,mask;
 
         for (;;) {
                 a = read_word_at_a_time(cs);
                 b = load_unaligned_zeropad(ct);
                 if (tcount < sizeof(unsigned long))
                         break;
                 if (unlikely(a != b))
                         return 1;
                 cs += sizeof(unsigned long);
                 ct += sizeof(unsigned long);
                 tcount -= sizeof(unsigned long);
                 if (!tcount)
                         return 0;
         }
         mask = bytemask_from_count(tcount);
         return unlikely(!!((a ^ b) & mask));
 }
 
 #else
 
 static inline int dentry_string_cmp(const unsigned char *cs, const unsigned char *ct, unsigned tcount)
 {
         do {
                 if (*cs != *ct)
                         return 1;
                 cs++;
                 ct++;
                 tcount--;
         } while (tcount);
         return 0;
 }
 
 #endif
 
 static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *ct, unsigned tcount)
 {
         /*
          * Be careful about RCU walk racing with rename:
          * use 'READ_ONCE' to fetch the name pointer.
          *
          * NOTE! Even if a rename will mean that the length
          * was not loaded atomically, we don't care. The
          * RCU walk will check the sequence count eventually,
          * and catch it. And we won't overrun the buffer,
          * because we're reading the name pointer atomically,
          * and a dentry name is guaranteed to be properly
          * terminated with a NUL byte.
          *
          * End result: even if 'len' is wrong, we'll exit
          * early because the data cannot match (there can
          * be no NUL in the ct/tcount data)
          */
         const unsigned char *cs = READ_ONCE(dentry->d_name.name);
 
         return dentry_string_cmp(cs, ct, tcount);
 }
 
 struct external_name {
         union {
                 atomic_t count;
                 struct rcu_head head;
         } u;
         unsigned char name[];
 };
 
 static inline struct external_name *external_name(struct dentry *dentry)
 {
         return container_of(dentry->d_name.name, struct external_name, name[0]);
 }
 
 static void __d_free(struct rcu_head *head)
 {
         struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
 
         kmem_cache_free(dentry_cache, dentry); 
 }
 
 static void __d_free_external(struct rcu_head *head)
 {
         struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
         kfree(external_name(dentry));
         kmem_cache_free(dentry_cache, dentry);
 }
 
 static inline int dname_external(const struct dentry *dentry)
 {
         return dentry->d_name.name != dentry->d_iname;
 }
 
 void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry)
 {
         spin_lock(&dentry->d_lock);
         name->name = dentry->d_name;
         if (unlikely(dname_external(dentry))) {
                 atomic_inc(&external_name(dentry)->u.count);
         } else {
                 memcpy(name->inline_name, dentry->d_iname,
                        dentry->d_name.len + 1);
                 name->name.name = name->inline_name;
         }
         spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(take_dentry_name_snapshot);
 
 void release_dentry_name_snapshot(struct name_snapshot *name)
 {
         if (unlikely(name->name.name != name->inline_name)) {
                 struct external_name *p;
                 p = container_of(name->name.name, struct external_name, name[0]);
                 if (unlikely(atomic_dec_and_test(&p->u.count)))
                         kfree_rcu(p, u.head);
         }
 }
 EXPORT_SYMBOL(release_dentry_name_snapshot);
 
 static inline void __d_set_inode_and_type(struct dentry *dentry,
                                           struct inode *inode,
                                           unsigned type_flags)
 {
         unsigned flags;
 
         dentry->d_inode = inode;
         flags = READ_ONCE(dentry->d_flags);
         flags &= ~DCACHE_ENTRY_TYPE;
         flags |= type_flags;
         smp_store_release(&dentry->d_flags, flags);
 }
 
 static inline void __d_clear_type_and_inode(struct dentry *dentry)
 {
         unsigned flags = READ_ONCE(dentry->d_flags);
 
         flags &= ~DCACHE_ENTRY_TYPE;
         WRITE_ONCE(dentry->d_flags, flags);
         dentry->d_inode = NULL;
         /*
          * The negative counter only tracks dentries on the LRU. Don't inc if
          * d_lru is on another list.
          */
         if ((flags & (DCACHE_LRU_LIST|DCACHE_SHRINK_LIST)) == DCACHE_LRU_LIST)
                 this_cpu_inc(nr_dentry_negative);
 }
 
 static void dentry_free(struct dentry *dentry)
 {
         WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
         if (unlikely(dname_external(dentry))) {
                 struct external_name *p = external_name(dentry);
                 if (likely(atomic_dec_and_test(&p->u.count))) {
                         call_rcu(&dentry->d_u.d_rcu, __d_free_external);
                         return;
                 }
         }
         /* if dentry was never visible to RCU, immediate free is OK */
         if (dentry->d_flags & DCACHE_NORCU)
                 __d_free(&dentry->d_u.d_rcu);
         else
                 call_rcu(&dentry->d_u.d_rcu, __d_free);
 }
 
 /*
  * Release the dentry's inode, using the filesystem
  * d_iput() operation if defined.
  */
 static void dentry_unlink_inode(struct dentry * dentry)
         __releases(dentry->d_lock)
         __releases(dentry->d_inode->i_lock)
 {
         struct inode *inode = dentry->d_inode;
 
         raw_write_seqcount_begin(&dentry->d_seq);
         __d_clear_type_and_inode(dentry);
         hlist_del_init(&dentry->d_u.d_alias);
         raw_write_seqcount_end(&dentry->d_seq);
         spin_unlock(&dentry->d_lock);
         spin_unlock(&inode->i_lock);
         if (!inode->i_nlink)
                 fsnotify_inoderemove(inode);
         if (dentry->d_op && dentry->d_op->d_iput)
                 dentry->d_op->d_iput(dentry, inode);
         else
                 iput(inode);
 }
 
 /*
  * The DCACHE_LRU_LIST bit is set whenever the 'd_lru' entry
  * is in use - which includes both the "real" per-superblock
  * LRU list _and_ the DCACHE_SHRINK_LIST use.
  *
  * The DCACHE_SHRINK_LIST bit is set whenever the dentry is
  * on the shrink list (ie not on the superblock LRU list).
  *
  * The per-cpu "nr_dentry_unused" counters are updated with
  * the DCACHE_LRU_LIST bit.
  *
  * The per-cpu "nr_dentry_negative" counters are only updated
  * when deleted from or added to the per-superblock LRU list, not
  * from/to the shrink list. That is to avoid an unneeded dec/inc
  * pair when moving from LRU to shrink list in select_collect().
  *
  * These helper functions make sure we always follow the
  * rules. d_lock must be held by the caller.
  */
 #define D_FLAG_VERIFY(dentry,x) WARN_ON_ONCE(((dentry)->d_flags & (DCACHE_LRU_LIST | DCACHE_SHRINK_LIST)) != (x))
 static void d_lru_add(struct dentry *dentry)
 {
         D_FLAG_VERIFY(dentry, 0);
         dentry->d_flags |= DCACHE_LRU_LIST;
         this_cpu_inc(nr_dentry_unused);
         if (d_is_negative(dentry))
                 this_cpu_inc(nr_dentry_negative);
         WARN_ON_ONCE(!list_lru_add_obj(
                         &dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
 static void d_lru_del(struct dentry *dentry)
 {
         D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
         dentry->d_flags &= ~DCACHE_LRU_LIST;
         this_cpu_dec(nr_dentry_unused);
         if (d_is_negative(dentry))
                 this_cpu_dec(nr_dentry_negative);
         WARN_ON_ONCE(!list_lru_del_obj(
                         &dentry->d_sb->s_dentry_lru, &dentry->d_lru));
 }
 
 static void d_shrink_del(struct dentry *dentry)
 {
         D_FLAG_VERIFY(dentry, DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
         list_del_init(&dentry->d_lru);
         dentry->d_flags &= ~(DCACHE_SHRINK_LIST | DCACHE_LRU_LIST);
         this_cpu_dec(nr_dentry_unused);
 }
 
 static void d_shrink_add(struct dentry *dentry, struct list_head *list)
 {
         D_FLAG_VERIFY(dentry, 0);
         list_add(&dentry->d_lru, list);
         dentry->d_flags |= DCACHE_SHRINK_LIST | DCACHE_LRU_LIST;
         this_cpu_inc(nr_dentry_unused);
 }
 
 /*
  * These can only be called under the global LRU lock, ie during the
  * callback for freeing the LRU list. "isolate" removes it from the
  * LRU lists entirely, while shrink_move moves it to the indicated
  * private list.
  */
 static void d_lru_isolate(struct list_lru_one *lru, struct dentry *dentry)
 {
         D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
         dentry->d_flags &= ~DCACHE_LRU_LIST;
         this_cpu_dec(nr_dentry_unused);
         if (d_is_negative(dentry))
                 this_cpu_dec(nr_dentry_negative);
         list_lru_isolate(lru, &dentry->d_lru);
 }
 
 static void d_lru_shrink_move(struct list_lru_one *lru, struct dentry *dentry,
                               struct list_head *list)
 {
         D_FLAG_VERIFY(dentry, DCACHE_LRU_LIST);
         dentry->d_flags |= DCACHE_SHRINK_LIST;
         if (d_is_negative(dentry))
                 this_cpu_dec(nr_dentry_negative);
         list_lru_isolate_move(lru, &dentry->d_lru, list);
 }
 
 static void ___d_drop(struct dentry *dentry)
 {
         struct hlist_bl_head *b;
         /*
          * Hashed dentries are normally on the dentry hashtable,
          * with the exception of those newly allocated by
          * d_obtain_root, which are always IS_ROOT:
          */
         if (unlikely(IS_ROOT(dentry)))
                 b = &dentry->d_sb->s_roots;
         else
                 b = d_hash(dentry->d_name.hash);
 
         hlist_bl_lock(b);
         __hlist_bl_del(&dentry->d_hash);
         hlist_bl_unlock(b);
 }
 
 void __d_drop(struct dentry *dentry)
 {
         if (!d_unhashed(dentry)) {
                 ___d_drop(dentry);
                 dentry->d_hash.pprev = NULL;
                 write_seqcount_invalidate(&dentry->d_seq);
         }
 }
 EXPORT_SYMBOL(__d_drop);
 
 /**
  * d_drop - drop a dentry
  * @dentry: dentry to drop
  *
  * d_drop() unhashes the entry from the parent dentry hashes, so that it won't
  * be found through a VFS lookup any more. Note that this is different from
  * deleting the dentry - d_delete will try to mark the dentry negative if
  * possible, giving a successful _negative_ lookup, while d_drop will
  * just make the cache lookup fail.
  *
  * d_drop() is used mainly for stuff that wants to invalidate a dentry for some
  * reason (NFS timeouts or autofs deletes).
  *
  * __d_drop requires dentry->d_lock
  *
  * ___d_drop doesn't mark dentry as "unhashed"
  * (dentry->d_hash.pprev will be LIST_POISON2, not NULL).
  */
 void d_drop(struct dentry *dentry)
 {
         spin_lock(&dentry->d_lock);
         __d_drop(dentry);
         spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(d_drop);
 
 static inline void dentry_unlist(struct dentry *dentry)
 {
         struct dentry *next;
         /*
          * Inform d_walk() and shrink_dentry_list() that we are no longer
          * attached to the dentry tree
          */
         dentry->d_flags |= DCACHE_DENTRY_KILLED;
         if (unlikely(hlist_unhashed(&dentry->d_sib)))
                 return;
         __hlist_del(&dentry->d_sib);
         /*
          * Cursors can move around the list of children.  While we'd been
          * a normal list member, it didn't matter - ->d_sib.next would've
          * been updated.  However, from now on it won't be and for the
          * things like d_walk() it might end up with a nasty surprise.
          * Normally d_walk() doesn't care about cursors moving around -
          * ->d_lock on parent prevents that and since a cursor has no children
          * of its own, we get through it without ever unlocking the parent.
          * There is one exception, though - if we ascend from a child that
          * gets killed as soon as we unlock it, the next sibling is found
          * using the value left in its ->d_sib.next.  And if _that_
          * pointed to a cursor, and cursor got moved (e.g. by lseek())
          * before d_walk() regains parent->d_lock, we'll end up skipping
          * everything the cursor had been moved past.
          *
          * Solution: make sure that the pointer left behind in ->d_sib.next
          * points to something that won't be moving around.  I.e. skip the
          * cursors.
          */
         while (dentry->d_sib.next) {
                 next = hlist_entry(dentry->d_sib.next, struct dentry, d_sib);
                 if (likely(!(next->d_flags & DCACHE_DENTRY_CURSOR)))
                         break;
                 dentry->d_sib.next = next->d_sib.next;
         }
 }
 
 static struct dentry *__dentry_kill(struct dentry *dentry)
 {
         struct dentry *parent = NULL;
         bool can_free = true;
 
         /*
          * The dentry is now unrecoverably dead to the world.
          */
         lockref_mark_dead(&dentry->d_lockref);
 
         /*
          * inform the fs via d_prune that this dentry is about to be
          * unhashed and destroyed.
          */
         if (dentry->d_flags & DCACHE_OP_PRUNE)
                 dentry->d_op->d_prune(dentry);
 
         if (dentry->d_flags & DCACHE_LRU_LIST) {
                 if (!(dentry->d_flags & DCACHE_SHRINK_LIST))
                         d_lru_del(dentry);
         }
         /* if it was on the hash then remove it */
         __d_drop(dentry);
         if (dentry->d_inode)
                 dentry_unlink_inode(dentry);
         else
                 spin_unlock(&dentry->d_lock);
         this_cpu_dec(nr_dentry);
         if (dentry->d_op && dentry->d_op->d_release)
                 dentry->d_op->d_release(dentry);
 
         cond_resched();
         /* now that it's negative, ->d_parent is stable */
         if (!IS_ROOT(dentry)) {
                 parent = dentry->d_parent;
                 spin_lock(&parent->d_lock);
         }
         spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
         dentry_unlist(dentry);
         if (dentry->d_flags & DCACHE_SHRINK_LIST)
                 can_free = false;
         spin_unlock(&dentry->d_lock);
         if (likely(can_free))
                 dentry_free(dentry);
         if (parent && --parent->d_lockref.count) {
                 spin_unlock(&parent->d_lock);
                 return NULL;
         }
         return parent;
 }
 
 /*
  * Lock a dentry for feeding it to __dentry_kill().
  * Called under rcu_read_lock() and dentry->d_lock; the former
  * guarantees that nothing we access will be freed under us.
  * Note that dentry is *not* protected from concurrent dentry_kill(),
  * d_delete(), etc.
  *
  * Return false if dentry is busy.  Otherwise, return true and have
  * that dentry's inode locked.
  */
 
 static bool lock_for_kill(struct dentry *dentry)
 {
         struct inode *inode = dentry->d_inode;
 
         if (unlikely(dentry->d_lockref.count))
                 return false;
 
         if (!inode || likely(spin_trylock(&inode->i_lock)))
                 return true;
 
         do {
                 spin_unlock(&dentry->d_lock);
                 spin_lock(&inode->i_lock);
                 spin_lock(&dentry->d_lock);
                 if (likely(inode == dentry->d_inode))
                         break;
                 spin_unlock(&inode->i_lock);
                 inode = dentry->d_inode;
         } while (inode);
         if (likely(!dentry->d_lockref.count))
                 return true;
         if (inode)
                 spin_unlock(&inode->i_lock);
         return false;
 }
 
 /*
  * Decide if dentry is worth retaining.  Usually this is called with dentry
  * locked; if not locked, we are more limited and might not be able to tell
  * without a lock.  False in this case means "punt to locked path and recheck".
  *
  * In case we aren't locked, these predicates are not "stable". However, it is
  * sufficient that at some point after we dropped the reference the dentry was
  * hashed and the flags had the proper value. Other dentry users may have
  * re-gotten a reference to the dentry and change that, but our work is done -
  * we can leave the dentry around with a zero refcount.
  */
 static inline bool retain_dentry(struct dentry *dentry, bool locked)
 {
         unsigned int d_flags;
 
         smp_rmb();
         d_flags = READ_ONCE(dentry->d_flags);
 
         // Unreachable? Nobody would be able to look it up, no point retaining
         if (unlikely(d_unhashed(dentry)))
                 return false;
 
         // Same if it's disconnected
         if (unlikely(d_flags & DCACHE_DISCONNECTED))
                 return false;
 
         // ->d_delete() might tell us not to bother, but that requires
         // ->d_lock; can't decide without it
         if (unlikely(d_flags & DCACHE_OP_DELETE)) {
                 if (!locked || dentry->d_op->d_delete(dentry))
                         return false;
         }
 
         // Explicitly told not to bother
         if (unlikely(d_flags & DCACHE_DONTCACHE))
                 return false;
 
         // At this point it looks like we ought to keep it.  We also might
         // need to do something - put it on LRU if it wasn't there already
         // and mark it referenced if it was on LRU, but not marked yet.
         // Unfortunately, both actions require ->d_lock, so in lockless
         // case we'd have to punt rather than doing those.
         if (unlikely(!(d_flags & DCACHE_LRU_LIST))) {
                 if (!locked)
                         return false;
                 d_lru_add(dentry);
         } else if (unlikely(!(d_flags & DCACHE_REFERENCED))) {
                 if (!locked)
                         return false;
                 dentry->d_flags |= DCACHE_REFERENCED;
         }
         return true;
 }
 
 void d_mark_dontcache(struct inode *inode)
 {
         struct dentry *de;
 
         spin_lock(&inode->i_lock);
         hlist_for_each_entry(de, &inode->i_dentry, d_u.d_alias) {
                 spin_lock(&de->d_lock);
                 de->d_flags |= DCACHE_DONTCACHE;
                 spin_unlock(&de->d_lock);
         }
         inode->i_state |= I_DONTCACHE;
         spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_mark_dontcache);
 
 /*
  * Try to do a lockless dput(), and return whether that was successful.
  *
  * If unsuccessful, we return false, having already taken the dentry lock.
  * In that case refcount is guaranteed to be zero and we have already
  * decided that it's not worth keeping around.
  *
  * The caller needs to hold the RCU read lock, so that the dentry is
  * guaranteed to stay around even if the refcount goes down to zero!
  */
 static inline bool fast_dput(struct dentry *dentry)
 {
         int ret;
 
         /*
          * try to decrement the lockref optimistically.
          */
         ret = lockref_put_return(&dentry->d_lockref);
 
         /*
          * If the lockref_put_return() failed due to the lock being held
          * by somebody else, the fast path has failed. We will need to
          * get the lock, and then check the count again.
          */
         if (unlikely(ret < 0)) {
                 spin_lock(&dentry->d_lock);
                 if (WARN_ON_ONCE(dentry->d_lockref.count <= 0)) {
                         spin_unlock(&dentry->d_lock);
                         return true;
                 }
                 dentry->d_lockref.count--;
                 goto locked;
         }
 
         /*
          * If we weren't the last ref, we're done.
          */
         if (ret)
                 return true;
 
         /*
          * Can we decide that decrement of refcount is all we needed without
          * taking the lock?  There's a very common case when it's all we need -
          * dentry looks like it ought to be retained and there's nothing else
          * to do.
          */
         if (retain_dentry(dentry, false))
                 return true;
 
         /*
          * Either not worth retaining or we can't tell without the lock.
          * Get the lock, then.  We've already decremented the refcount to 0,
          * but we'll need to re-check the situation after getting the lock.
          */
         spin_lock(&dentry->d_lock);
 
         /*
          * Did somebody else grab a reference to it in the meantime, and
          * we're no longer the last user after all? Alternatively, somebody
          * else could have killed it and marked it dead. Either way, we
          * don't need to do anything else.
          */
 locked:
         if (dentry->d_lockref.count || retain_dentry(dentry, true)) {
                 spin_unlock(&dentry->d_lock);
                 return true;
         }
         return false;
 }
 
 
 /* 
  * This is dput
  *
  * This is complicated by the fact that we do not want to put
  * dentries that are no longer on any hash chain on the unused
  * list: we'd much rather just get rid of them immediately.
  *
  * However, that implies that we have to traverse the dentry
  * tree upwards to the parents which might _also_ now be
  * scheduled for deletion (it may have been only waiting for
  * its last child to go away).
  *
  * This tail recursion is done by hand as we don't want to depend
  * on the compiler to always get this right (gcc generally doesn't).
  * Real recursion would eat up our stack space.
  */
 
 /*
  * dput - release a dentry
  * @dentry: dentry to release 
  *
  * Release a dentry. This will drop the usage count and if appropriate
  * call the dentry unlink method as well as removing it from the queues and
  * releasing its resources. If the parent dentries were scheduled for release
  * they too may now get deleted.
  */
 void dput(struct dentry *dentry)
 {
         if (!dentry)
                 return;
         might_sleep();
         rcu_read_lock();
         if (likely(fast_dput(dentry))) {
                 rcu_read_unlock();
                 return;
         }
         while (lock_for_kill(dentry)) {
                 rcu_read_unlock();
                 dentry = __dentry_kill(dentry);
                 if (!dentry)
                         return;
                 if (retain_dentry(dentry, true)) {
                         spin_unlock(&dentry->d_lock);
                         return;
                 }
                 rcu_read_lock();
         }
         rcu_read_unlock();
         spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(dput);
 
 static void to_shrink_list(struct dentry *dentry, struct list_head *list)
 __must_hold(&dentry->d_lock)
 {
         if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
                 if (dentry->d_flags & DCACHE_LRU_LIST)
                         d_lru_del(dentry);
                 d_shrink_add(dentry, list);
         }
 }
 
 void dput_to_list(struct dentry *dentry, struct list_head *list)
 {
         rcu_read_lock();
         if (likely(fast_dput(dentry))) {
                 rcu_read_unlock();
                 return;
         }
         rcu_read_unlock();
         to_shrink_list(dentry, list);
         spin_unlock(&dentry->d_lock);
 }
 
 struct dentry *dget_parent(struct dentry *dentry)
 {
         int gotref;
         struct dentry *ret;
         unsigned seq;
 
         /*
          * Do optimistic parent lookup without any
          * locking.
          */
         rcu_read_lock();
         seq = raw_seqcount_begin(&dentry->d_seq);
         ret = READ_ONCE(dentry->d_parent);
         gotref = lockref_get_not_zero(&ret->d_lockref);
         rcu_read_unlock();
         if (likely(gotref)) {
                 if (!read_seqcount_retry(&dentry->d_seq, seq))
                         return ret;
                 dput(ret);
         }
 
 repeat:
         /*
          * Don't need rcu_dereference because we re-check it was correct under
          * the lock.
          */
         rcu_read_lock();
         ret = dentry->d_parent;
         spin_lock(&ret->d_lock);
         if (unlikely(ret != dentry->d_parent)) {
                 spin_unlock(&ret->d_lock);
                 rcu_read_unlock();
                 goto repeat;
         }
         rcu_read_unlock();
         BUG_ON(!ret->d_lockref.count);
         ret->d_lockref.count++;
         spin_unlock(&ret->d_lock);
         return ret;
 }
 EXPORT_SYMBOL(dget_parent);
 
 static struct dentry * __d_find_any_alias(struct inode *inode)
 {
         struct dentry *alias;
 
         if (hlist_empty(&inode->i_dentry))
                 return NULL;
         alias = hlist_entry(inode->i_dentry.first, struct dentry, d_u.d_alias);
         lockref_get(&alias->d_lockref);
         return alias;
 }
 
 /**
  * d_find_any_alias - find any alias for a given inode
  * @inode: inode to find an alias for
  *
  * If any aliases exist for the given inode, take and return a
  * reference for one of them.  If no aliases exist, return %NULL.
  */
 struct dentry *d_find_any_alias(struct inode *inode)
 {
         struct dentry *de;
 
         spin_lock(&inode->i_lock);
         de = __d_find_any_alias(inode);
         spin_unlock(&inode->i_lock);
         return de;
 }
 EXPORT_SYMBOL(d_find_any_alias);
 
 static struct dentry *__d_find_alias(struct inode *inode)
 {
         struct dentry *alias;
 
         if (S_ISDIR(inode->i_mode))
                 return __d_find_any_alias(inode);
 
         hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
                 spin_lock(&alias->d_lock);
                 if (!d_unhashed(alias)) {
                         dget_dlock(alias);
                         spin_unlock(&alias->d_lock);
                         return alias;
                 }
                 spin_unlock(&alias->d_lock);
         }
         return NULL;
 }
 
 /**
  * d_find_alias - grab a hashed alias of inode
  * @inode: inode in question
  *
  * If inode has a hashed alias, or is a directory and has any alias,
  * acquire the reference to alias and return it. Otherwise return NULL.
  * Notice that if inode is a directory there can be only one alias and
  * it can be unhashed only if it has no children, or if it is the root
  * of a filesystem, or if the directory was renamed and d_revalidate
  * was the first vfs operation to notice.
  *
  * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
  * any other hashed alias over that one.
  */
 struct dentry *d_find_alias(struct inode *inode)
 {
         struct dentry *de = NULL;
 
         if (!hlist_empty(&inode->i_dentry)) {
                 spin_lock(&inode->i_lock);
                 de = __d_find_alias(inode);
                 spin_unlock(&inode->i_lock);
         }
         return de;
 }
 EXPORT_SYMBOL(d_find_alias);
 
 /*
  *  Caller MUST be holding rcu_read_lock() and be guaranteed
  *  that inode won't get freed until rcu_read_unlock().
  */
 struct dentry *d_find_alias_rcu(struct inode *inode)
 {
         struct hlist_head *l = &inode->i_dentry;
         struct dentry *de = NULL;
 
         spin_lock(&inode->i_lock);
         // ->i_dentry and ->i_rcu are colocated, but the latter won't be
         // used without having I_FREEING set, which means no aliases left
         if (likely(!(inode->i_state & I_FREEING) && !hlist_empty(l))) {
                 if (S_ISDIR(inode->i_mode)) {
                         de = hlist_entry(l->first, struct dentry, d_u.d_alias);
                 } else {
                         hlist_for_each_entry(de, l, d_u.d_alias)
                                 if (!d_unhashed(de))
                                         break;
                 }
         }
         spin_unlock(&inode->i_lock);
         return de;
 }
 
 /*
  *      Try to kill dentries associated with this inode.
  * WARNING: you must own a reference to inode.
  */
 void d_prune_aliases(struct inode *inode)
 {
         LIST_HEAD(dispose);
         struct dentry *dentry;
 
         spin_lock(&inode->i_lock);
         hlist_for_each_entry(dentry, &inode->i_dentry, d_u.d_alias) {
                 spin_lock(&dentry->d_lock);
                 if (!dentry->d_lockref.count)
                         to_shrink_list(dentry, &dispose);
                 spin_unlock(&dentry->d_lock);
         }
         spin_unlock(&inode->i_lock);
         shrink_dentry_list(&dispose);
 }
 EXPORT_SYMBOL(d_prune_aliases);
 
 static inline void shrink_kill(struct dentry *victim)
 {
         do {
                 rcu_read_unlock();
                 victim = __dentry_kill(victim);
                 rcu_read_lock();
         } while (victim && lock_for_kill(victim));
         rcu_read_unlock();
         if (victim)
                 spin_unlock(&victim->d_lock);
 }
 
 void shrink_dentry_list(struct list_head *list)
 {
         while (!list_empty(list)) {
                 struct dentry *dentry;
 
                 dentry = list_entry(list->prev, struct dentry, d_lru);
                 spin_lock(&dentry->d_lock);
                 rcu_read_lock();
                 if (!lock_for_kill(dentry)) {
                         bool can_free;
                         rcu_read_unlock();
                         d_shrink_del(dentry);
                         can_free = dentry->d_flags & DCACHE_DENTRY_KILLED;
                         spin_unlock(&dentry->d_lock);
                         if (can_free)
                                 dentry_free(dentry);
                         continue;
                 }
                 d_shrink_del(dentry);
                 shrink_kill(dentry);
         }
 }
 
 static enum lru_status dentry_lru_isolate(struct list_head *item,
                 struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
 {
         struct list_head *freeable = arg;
         struct dentry   *dentry = container_of(item, struct dentry, d_lru);
 
 
         /*
          * we are inverting the lru lock/dentry->d_lock here,
          * so use a trylock. If we fail to get the lock, just skip
          * it
          */
         if (!spin_trylock(&dentry->d_lock))
                 return LRU_SKIP;
 
         /*
          * Referenced dentries are still in use. If they have active
          * counts, just remove them from the LRU. Otherwise give them
          * another pass through the LRU.
          */
         if (dentry->d_lockref.count) {
                 d_lru_isolate(lru, dentry);
                 spin_unlock(&dentry->d_lock);
                 return LRU_REMOVED;
         }
 
         if (dentry->d_flags & DCACHE_REFERENCED) {
                 dentry->d_flags &= ~DCACHE_REFERENCED;
                 spin_unlock(&dentry->d_lock);
 
                 /*
                  * The list move itself will be made by the common LRU code. At
                  * this point, we've dropped the dentry->d_lock but keep the
                  * lru lock. This is safe to do, since every list movement is
                  * protected by the lru lock even if both locks are held.
                  *
                  * This is guaranteed by the fact that all LRU management
                  * functions are intermediated by the LRU API calls like
                  * list_lru_add_obj and list_lru_del_obj. List movement in this file
                  * only ever occur through this functions or through callbacks
                  * like this one, that are called from the LRU API.
                  *
                  * The only exceptions to this are functions like
                  * shrink_dentry_list, and code that first checks for the
                  * DCACHE_SHRINK_LIST flag.  Those are guaranteed to be
                  * operating only with stack provided lists after they are
                  * properly isolated from the main list.  It is thus, always a
                  * local access.
                  */
                 return LRU_ROTATE;
         }
 
         d_lru_shrink_move(lru, dentry, freeable);
         spin_unlock(&dentry->d_lock);
 
         return LRU_REMOVED;
 }
 
 /**
  * prune_dcache_sb - shrink the dcache
  * @sb: superblock
  * @sc: shrink control, passed to list_lru_shrink_walk()
  *
  * Attempt to shrink the superblock dcache LRU by @sc->nr_to_scan entries. This
  * is done when we need more memory and called from the superblock shrinker
  * function.
  *
  * This function may fail to free any resources if all the dentries are in
  * use.
  */
 long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc)
 {
         LIST_HEAD(dispose);
         long freed;
 
         freed = list_lru_shrink_walk(&sb->s_dentry_lru, sc,
                                      dentry_lru_isolate, &dispose);
         shrink_dentry_list(&dispose);
         return freed;
 }
 
 static enum lru_status dentry_lru_isolate_shrink(struct list_head *item,
                 struct list_lru_one *lru, spinlock_t *lru_lock, void *arg)
 {
         struct list_head *freeable = arg;
         struct dentry   *dentry = container_of(item, struct dentry, d_lru);
 
         /*
          * we are inverting the lru lock/dentry->d_lock here,
          * so use a trylock. If we fail to get the lock, just skip
          * it
          */
         if (!spin_trylock(&dentry->d_lock))
                 return LRU_SKIP;
 
         d_lru_shrink_move(lru, dentry, freeable);
         spin_unlock(&dentry->d_lock);
 
         return LRU_REMOVED;
 }
 
 
 /**
  * shrink_dcache_sb - shrink dcache for a superblock
  * @sb: superblock
  *
  * Shrink the dcache for the specified super block. This is used to free
  * the dcache before unmounting a file system.
  */
 void shrink_dcache_sb(struct super_block *sb)
 {
         do {
                 LIST_HEAD(dispose);
 
                 list_lru_walk(&sb->s_dentry_lru,
                         dentry_lru_isolate_shrink, &dispose, 1024);
                 shrink_dentry_list(&dispose);
         } while (list_lru_count(&sb->s_dentry_lru) > 0);
 }
 EXPORT_SYMBOL(shrink_dcache_sb);
 
 /**
  * enum d_walk_ret - action to talke during tree walk
  * @D_WALK_CONTINUE:    contrinue walk
  * @D_WALK_QUIT:        quit walk
  * @D_WALK_NORETRY:     quit when retry is needed
  * @D_WALK_SKIP:        skip this dentry and its children
  */
 enum d_walk_ret {
         D_WALK_CONTINUE,
         D_WALK_QUIT,
         D_WALK_NORETRY,
         D_WALK_SKIP,
 };
 
 /**
  * d_walk - walk the dentry tree
  * @parent:     start of walk
  * @data:       data passed to @enter() and @finish()
  * @enter:      callback when first entering the dentry
  *
  * The @enter() callbacks are called with d_lock held.
  */
 static void d_walk(struct dentry *parent, void *data,
                    enum d_walk_ret (*enter)(void *, struct dentry *))
 {
         struct dentry *this_parent, *dentry;
         unsigned seq = 0;
         enum d_walk_ret ret;
         bool retry = true;
 
 again:
         read_seqbegin_or_lock(&rename_lock, &seq);
         this_parent = parent;
         spin_lock(&this_parent->d_lock);
 
         ret = enter(data, this_parent);
         switch (ret) {
         case D_WALK_CONTINUE:
                 break;
         case D_WALK_QUIT:
         case D_WALK_SKIP:
                 goto out_unlock;
         case D_WALK_NORETRY:
                 retry = false;
                 break;
         }
 repeat:
         dentry = d_first_child(this_parent);
 resume:
         hlist_for_each_entry_from(dentry, d_sib) {
                 if (unlikely(dentry->d_flags & DCACHE_DENTRY_CURSOR))
                         continue;
 
                 spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 
                 ret = enter(data, dentry);
                 switch (ret) {
                 case D_WALK_CONTINUE:
                         break;
                 case D_WALK_QUIT:
                         spin_unlock(&dentry->d_lock);
                         goto out_unlock;
                 case D_WALK_NORETRY:
                         retry = false;
                         break;
                 case D_WALK_SKIP:
                         spin_unlock(&dentry->d_lock);
                         continue;
                 }
 
                 if (!hlist_empty(&dentry->d_children)) {
                         spin_unlock(&this_parent->d_lock);
                         spin_release(&dentry->d_lock.dep_map, _RET_IP_);
                         this_parent = dentry;
                         spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
                         goto repeat;
                 }
                 spin_unlock(&dentry->d_lock);
         }
         /*
          * All done at this level ... ascend and resume the search.
          */
         rcu_read_lock();
 ascend:
         if (this_parent != parent) {
                 dentry = this_parent;
                 this_parent = dentry->d_parent;
 
                 spin_unlock(&dentry->d_lock);
                 spin_lock(&this_parent->d_lock);
 
                 /* might go back up the wrong parent if we have had a rename. */
                 if (need_seqretry(&rename_lock, seq))
                         goto rename_retry;
                 /* go into the first sibling still alive */
                 hlist_for_each_entry_continue(dentry, d_sib) {
                         if (likely(!(dentry->d_flags & DCACHE_DENTRY_KILLED))) {
                                 rcu_read_unlock();
                                 goto resume;
                         }
                 }
                 goto ascend;
         }
         if (need_seqretry(&rename_lock, seq))
                 goto rename_retry;
         rcu_read_unlock();
 
 out_unlock:
         spin_unlock(&this_parent->d_lock);
         done_seqretry(&rename_lock, seq);
         return;
 
 rename_retry:
         spin_unlock(&this_parent->d_lock);
         rcu_read_unlock();
         BUG_ON(seq & 1);
         if (!retry)
                 return;
         seq = 1;
         goto again;
 }
 
 struct check_mount {
         struct vfsmount *mnt;
         unsigned int mounted;
 };
 
 static enum d_walk_ret path_check_mount(void *data, struct dentry *dentry)
 {
         struct check_mount *info = data;
         struct path path = { .mnt = info->mnt, .dentry = dentry };
 
         if (likely(!d_mountpoint(dentry)))
                 return D_WALK_CONTINUE;
         if (__path_is_mountpoint(&path)) {
                 info->mounted = 1;
                 return D_WALK_QUIT;
         }
         return D_WALK_CONTINUE;
 }
 
 /**
  * path_has_submounts - check for mounts over a dentry in the
  *                      current namespace.
  * @parent: path to check.
  *
  * Return true if the parent or its subdirectories contain
  * a mount point in the current namespace.
  */
 int path_has_submounts(const struct path *parent)
 {
         struct check_mount data = { .mnt = parent->mnt, .mounted = 0 };
 
         read_seqlock_excl(&mount_lock);
         d_walk(parent->dentry, &data, path_check_mount);
         read_sequnlock_excl(&mount_lock);
 
         return data.mounted;
 }
 EXPORT_SYMBOL(path_has_submounts);
 
 /*
  * Called by mount code to set a mountpoint and check if the mountpoint is
  * reachable (e.g. NFS can unhash a directory dentry and then the complete
  * subtree can become unreachable).
  *
  * Only one of d_invalidate() and d_set_mounted() must succeed.  For
  * this reason take rename_lock and d_lock on dentry and ancestors.
  */
 int d_set_mounted(struct dentry *dentry)
 {
         struct dentry *p;
         int ret = -ENOENT;
         write_seqlock(&rename_lock);
         for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) {
                 /* Need exclusion wrt. d_invalidate() */
                 spin_lock(&p->d_lock);
                 if (unlikely(d_unhashed(p))) {
                         spin_unlock(&p->d_lock);
                         goto out;
                 }
                 spin_unlock(&p->d_lock);
         }
         spin_lock(&dentry->d_lock);
         if (!d_unlinked(dentry)) {
                 ret = -EBUSY;
                 if (!d_mountpoint(dentry)) {
                         dentry->d_flags |= DCACHE_MOUNTED;
                         ret = 0;
                 }
         }
         spin_unlock(&dentry->d_lock);
 out:
         write_sequnlock(&rename_lock);
         return ret;
 }
 
 /*
  * Search the dentry child list of the specified parent,
  * and move any unused dentries to the end of the unused
  * list for prune_dcache(). We descend to the next level
  * whenever the d_children list is non-empty and continue
  * searching.
  *
  * It returns zero iff there are no unused children,
  * otherwise  it returns the number of children moved to
  * the end of the unused list. This may not be the total
  * number of unused children, because select_parent can
  * drop the lock and return early due to latency
  * constraints.
  */
 
 struct select_data {
         struct dentry *start;
         union {
                 long found;
                 struct dentry *victim;
         };
         struct list_head dispose;
 };
 
 static enum d_walk_ret select_collect(void *_data, struct dentry *dentry)
 {
         struct select_data *data = _data;
         enum d_walk_ret ret = D_WALK_CONTINUE;
 
         if (data->start == dentry)
                 goto out;
 
         if (dentry->d_flags & DCACHE_SHRINK_LIST) {
                 data->found++;
         } else if (!dentry->d_lockref.count) {
                 to_shrink_list(dentry, &data->dispose);
                 data->found++;
         } else if (dentry->d_lockref.count < 0) {
                 data->found++;
         }
         /*
          * We can return to the caller if we have found some (this
          * ensures forward progress). We'll be coming back to find
          * the rest.
          */
         if (!list_empty(&data->dispose))
                 ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
 out:
         return ret;
 }
 
 static enum d_walk_ret select_collect2(void *_data, struct dentry *dentry)
 {
         struct select_data *data = _data;
         enum d_walk_ret ret = D_WALK_CONTINUE;
 
         if (data->start == dentry)
                 goto out;
 
         if (!dentry->d_lockref.count) {
                 if (dentry->d_flags & DCACHE_SHRINK_LIST) {
                         rcu_read_lock();
                         data->victim = dentry;
                         return D_WALK_QUIT;
                 }
                 to_shrink_list(dentry, &data->dispose);
         }
         /*
          * We can return to the caller if we have found some (this
          * ensures forward progress). We'll be coming back to find
          * the rest.
          */
         if (!list_empty(&data->dispose))
                 ret = need_resched() ? D_WALK_QUIT : D_WALK_NORETRY;
 out:
         return ret;
 }
 
 /**
  * shrink_dcache_parent - prune dcache
  * @parent: parent of entries to prune
  *
  * Prune the dcache to remove unused children of the parent dentry.
  */
 void shrink_dcache_parent(struct dentry *parent)
 {
         for (;;) {
                 struct select_data data = {.start = parent};
 
                 INIT_LIST_HEAD(&data.dispose);
                 d_walk(parent, &data, select_collect);
 
                 if (!list_empty(&data.dispose)) {
                         shrink_dentry_list(&data.dispose);
                         continue;
                 }
 
                 cond_resched();
                 if (!data.found)
                         break;
                 data.victim = NULL;
                 d_walk(parent, &data, select_collect2);
                 if (data.victim) {
                         spin_lock(&data.victim->d_lock);
                         if (!lock_for_kill(data.victim)) {
                                 spin_unlock(&data.victim->d_lock);
                                 rcu_read_unlock();
                         } else {
                                 shrink_kill(data.victim);
                         }
                 }
                 if (!list_empty(&data.dispose))
                         shrink_dentry_list(&data.dispose);
         }
 }
 EXPORT_SYMBOL(shrink_dcache_parent);
 
 static enum d_walk_ret umount_check(void *_data, struct dentry *dentry)
 {
         /* it has busy descendents; complain about those instead */
         if (!hlist_empty(&dentry->d_children))
                 return D_WALK_CONTINUE;
 
         /* root with refcount 1 is fine */
         if (dentry == _data && dentry->d_lockref.count == 1)
                 return D_WALK_CONTINUE;
 
         WARN(1, "BUG: Dentry %p{i=%lx,n=%pd} "
                         " still in use (%d) [unmount of %s %s]\n",
                        dentry,
                        dentry->d_inode ?
                        dentry->d_inode->i_ino : 0UL,
                        dentry,
                        dentry->d_lockref.count,
                        dentry->d_sb->s_type->name,
                        dentry->d_sb->s_id);
         return D_WALK_CONTINUE;
 }
 
 static void do_one_tree(struct dentry *dentry)
 {
         shrink_dcache_parent(dentry);
         d_walk(dentry, dentry, umount_check);
         d_drop(dentry);
         dput(dentry);
 }
 
 /*
  * destroy the dentries attached to a superblock on unmounting
  */
 void shrink_dcache_for_umount(struct super_block *sb)
 {
         struct dentry *dentry;
 
         rwsem_assert_held_write(&sb->s_umount);
 
         dentry = sb->s_root;
         sb->s_root = NULL;
         do_one_tree(dentry);
 
         while (!hlist_bl_empty(&sb->s_roots)) {
                 dentry = dget(hlist_bl_entry(hlist_bl_first(&sb->s_roots), struct dentry, d_hash));
                 do_one_tree(dentry);
         }
 }
 
 static enum d_walk_ret find_submount(void *_data, struct dentry *dentry)
 {
         struct dentry **victim = _data;
         if (d_mountpoint(dentry)) {
                 *victim = dget_dlock(dentry);
                 return D_WALK_QUIT;
         }
         return D_WALK_CONTINUE;
 }
 
 /**
  * d_invalidate - detach submounts, prune dcache, and drop
  * @dentry: dentry to invalidate (aka detach, prune and drop)
  */
 void d_invalidate(struct dentry *dentry)
 {
         bool had_submounts = false;
         spin_lock(&dentry->d_lock);
         if (d_unhashed(dentry)) {
                 spin_unlock(&dentry->d_lock);
                 return;
         }
         __d_drop(dentry);
         spin_unlock(&dentry->d_lock);
 
         /* Negative dentries can be dropped without further checks */
         if (!dentry->d_inode)
                 return;
 
         shrink_dcache_parent(dentry);
         for (;;) {
                 struct dentry *victim = NULL;
                 d_walk(dentry, &victim, find_submount);
                 if (!victim) {
                         if (had_submounts)
                                 shrink_dcache_parent(dentry);
                         return;
                 }
                 had_submounts = true;
                 detach_mounts(victim);
                 dput(victim);
         }
 }
 EXPORT_SYMBOL(d_invalidate);
 
 /**
  * __d_alloc    -       allocate a dcache entry
  * @sb: filesystem it will belong to
  * @name: qstr of the name
  *
  * Allocates a dentry. It returns %NULL if there is insufficient memory
  * available. On a success the dentry is returned. The name passed in is
  * copied and the copy passed in may be reused after this call.
  */
  
 static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 {
         struct dentry *dentry;
         char *dname;
         int err;
 
         dentry = kmem_cache_alloc_lru(dentry_cache, &sb->s_dentry_lru,
                                       GFP_KERNEL);
         if (!dentry)
                 return NULL;
 
         /*
          * We guarantee that the inline name is always NUL-terminated.
          * This way the memcpy() done by the name switching in rename
          * will still always have a NUL at the end, even if we might
          * be overwriting an internal NUL character
          */
         dentry->d_iname[DNAME_INLINE_LEN-1] = 0;
         if (unlikely(!name)) {
                 name = &slash_name;
                 dname = dentry->d_iname;
         } else if (name->len > DNAME_INLINE_LEN-1) {
                 size_t size = offsetof(struct external_name, name[1]);
                 struct external_name *p = kmalloc(size + name->len,
                                                   GFP_KERNEL_ACCOUNT |
                                                   __GFP_RECLAIMABLE);
                 if (!p) {
                         kmem_cache_free(dentry_cache, dentry); 
                         return NULL;
                 }
                 atomic_set(&p->u.count, 1);
                 dname = p->name;
         } else  {
                 dname = dentry->d_iname;
         }       
 
         dentry->d_name.len = name->len;
         dentry->d_name.hash = name->hash;
         memcpy(dname, name->name, name->len);
         dname[name->len] = 0;
 
         /* Make sure we always see the terminating NUL character */
         smp_store_release(&dentry->d_name.name, dname); /* ^^^ */
 
         dentry->d_lockref.count = 1;
         dentry->d_flags = 0;
         spin_lock_init(&dentry->d_lock);
         seqcount_spinlock_init(&dentry->d_seq, &dentry->d_lock);
         dentry->d_inode = NULL;
         dentry->d_parent = dentry;
         dentry->d_sb = sb;
         dentry->d_op = NULL;
         dentry->d_fsdata = NULL;
         INIT_HLIST_BL_NODE(&dentry->d_hash);
         INIT_LIST_HEAD(&dentry->d_lru);
         INIT_HLIST_HEAD(&dentry->d_children);
         INIT_HLIST_NODE(&dentry->d_u.d_alias);
         INIT_HLIST_NODE(&dentry->d_sib);
         d_set_d_op(dentry, dentry->d_sb->s_d_op);
 
         if (dentry->d_op && dentry->d_op->d_init) {
                 err = dentry->d_op->d_init(dentry);
                 if (err) {
                         if (dname_external(dentry))
                                 kfree(external_name(dentry));
                         kmem_cache_free(dentry_cache, dentry);
                         return NULL;
                 }
         }
 
         this_cpu_inc(nr_dentry);
 
         return dentry;
 }
 
 /**
  * d_alloc      -       allocate a dcache entry
  * @parent: parent of entry to allocate
  * @name: qstr of the name
  *
  * Allocates a dentry. It returns %NULL if there is insufficient memory
  * available. On a success the dentry is returned. The name passed in is
  * copied and the copy passed in may be reused after this call.
  */
 struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
 {
         struct dentry *dentry = __d_alloc(parent->d_sb, name);
         if (!dentry)
                 return NULL;
         spin_lock(&parent->d_lock);
         /*
          * don't need child lock because it is not subject
          * to concurrency here
          */
         dentry->d_parent = dget_dlock(parent);
         hlist_add_head(&dentry->d_sib, &parent->d_children);
         spin_unlock(&parent->d_lock);
 
         return dentry;
 }
 EXPORT_SYMBOL(d_alloc);
 
 struct dentry *d_alloc_anon(struct super_block *sb)
 {
         return __d_alloc(sb, NULL);
 }
 EXPORT_SYMBOL(d_alloc_anon);
 
 struct dentry *d_alloc_cursor(struct dentry * parent)
 {
         struct dentry *dentry = d_alloc_anon(parent->d_sb);
         if (dentry) {
                 dentry->d_flags |= DCACHE_DENTRY_CURSOR;
                 dentry->d_parent = dget(parent);
         }
         return dentry;
 }
 
 /**
  * d_alloc_pseudo - allocate a dentry (for lookup-less filesystems)
  * @sb: the superblock
  * @name: qstr of the name
  *
  * For a filesystem that just pins its dentries in memory and never
  * performs lookups at all, return an unhashed IS_ROOT dentry.
  * This is used for pipes, sockets et.al. - the stuff that should
  * never be anyone's children or parents.  Unlike all other
  * dentries, these will not have RCU delay between dropping the
  * last reference and freeing them.
  *
  * The only user is alloc_file_pseudo() and that's what should
  * be considered a public interface.  Don't use directly.
  */
 struct dentry *d_alloc_pseudo(struct super_block *sb, const struct qstr *name)
 {
         static const struct dentry_operations anon_ops = {
                 .d_dname = simple_dname
         };
         struct dentry *dentry = __d_alloc(sb, name);
         if (likely(dentry)) {
                 dentry->d_flags |= DCACHE_NORCU;
                 if (!sb->s_d_op)
                         d_set_d_op(dentry, &anon_ops);
         }
         return dentry;
 }
 
 struct dentry *d_alloc_name(struct dentry *parent, const char *name)
 {
         struct qstr q;
 
         q.name = name;
         q.hash_len = hashlen_string(parent, name);
         return d_alloc(parent, &q);
 }
 EXPORT_SYMBOL(d_alloc_name);
 
 void d_set_d_op(struct dentry *dentry, const struct dentry_operations *op)
 {
         WARN_ON_ONCE(dentry->d_op);
         WARN_ON_ONCE(dentry->d_flags & (DCACHE_OP_HASH  |
                                 DCACHE_OP_COMPARE       |
                                 DCACHE_OP_REVALIDATE    |
                                 DCACHE_OP_WEAK_REVALIDATE       |
                                 DCACHE_OP_DELETE        |
                                 DCACHE_OP_REAL));
         dentry->d_op = op;
         if (!op)
                 return;
         if (op->d_hash)
                 dentry->d_flags |= DCACHE_OP_HASH;
         if (op->d_compare)
                 dentry->d_flags |= DCACHE_OP_COMPARE;
         if (op->d_revalidate)
                 dentry->d_flags |= DCACHE_OP_REVALIDATE;
         if (op->d_weak_revalidate)
                 dentry->d_flags |= DCACHE_OP_WEAK_REVALIDATE;
         if (op->d_delete)
                 dentry->d_flags |= DCACHE_OP_DELETE;
         if (op->d_prune)
                 dentry->d_flags |= DCACHE_OP_PRUNE;
         if (op->d_real)
                 dentry->d_flags |= DCACHE_OP_REAL;
 
 }
 EXPORT_SYMBOL(d_set_d_op);
 
 static unsigned d_flags_for_inode(struct inode *inode)
 {
         unsigned add_flags = DCACHE_REGULAR_TYPE;
 
         if (!inode)
                 return DCACHE_MISS_TYPE;
 
         if (S_ISDIR(inode->i_mode)) {
                 add_flags = DCACHE_DIRECTORY_TYPE;
                 if (unlikely(!(inode->i_opflags & IOP_LOOKUP))) {
                         if (unlikely(!inode->i_op->lookup))
                                 add_flags = DCACHE_AUTODIR_TYPE;
                         else
                                 inode->i_opflags |= IOP_LOOKUP;
                 }
                 goto type_determined;
         }
 
         if (unlikely(!(inode->i_opflags & IOP_NOFOLLOW))) {
                 if (unlikely(inode->i_op->get_link)) {
                         add_flags = DCACHE_SYMLINK_TYPE;
                         goto type_determined;
                 }
                 inode->i_opflags |= IOP_NOFOLLOW;
         }
 
         if (unlikely(!S_ISREG(inode->i_mode)))
                 add_flags = DCACHE_SPECIAL_TYPE;
 
 type_determined:
         if (unlikely(IS_AUTOMOUNT(inode)))
                 add_flags |= DCACHE_NEED_AUTOMOUNT;
         return add_flags;
 }
 
 static void __d_instantiate(struct dentry *dentry, struct inode *inode)
 {
         unsigned add_flags = d_flags_for_inode(inode);
         WARN_ON(d_in_lookup(dentry));
 
         spin_lock(&dentry->d_lock);
         /*
          * The negative counter only tracks dentries on the LRU. Don't dec if
          * d_lru is on another list.
          */
         if ((dentry->d_flags &
              (DCACHE_LRU_LIST|DCACHE_SHRINK_LIST)) == DCACHE_LRU_LIST)
                 this_cpu_dec(nr_dentry_negative);
         hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
         raw_write_seqcount_begin(&dentry->d_seq);
         __d_set_inode_and_type(dentry, inode, add_flags);
         raw_write_seqcount_end(&dentry->d_seq);
         fsnotify_update_flags(dentry);
         spin_unlock(&dentry->d_lock);
 }
 
 /**
  * d_instantiate - fill in inode information for a dentry
  * @entry: dentry to complete
  * @inode: inode to attach to this dentry
  *
  * Fill in inode information in the entry.
  *
  * This turns negative dentries into productive full members
  * of society.
  *
  * NOTE! This assumes that the inode count has been incremented
  * (or otherwise set) by the caller to indicate that it is now
  * in use by the dcache.
  */
  
 void d_instantiate(struct dentry *entry, struct inode * inode)
 {
         BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
         if (inode) {
                 security_d_instantiate(entry, inode);
                 spin_lock(&inode->i_lock);
                 __d_instantiate(entry, inode);
                 spin_unlock(&inode->i_lock);
         }
 }
 EXPORT_SYMBOL(d_instantiate);
 
 /*
  * This should be equivalent to d_instantiate() + unlock_new_inode(),
  * with lockdep-related part of unlock_new_inode() done before
  * anything else.  Use that instead of open-coding d_instantiate()/
  * unlock_new_inode() combinations.
  */
 void d_instantiate_new(struct dentry *entry, struct inode *inode)
 {
         BUG_ON(!hlist_unhashed(&entry->d_u.d_alias));
         BUG_ON(!inode);
         lockdep_annotate_inode_mutex_key(inode);
         security_d_instantiate(entry, inode);
         spin_lock(&inode->i_lock);
         __d_instantiate(entry, inode);
         WARN_ON(!(inode->i_state & I_NEW));
         inode->i_state &= ~I_NEW & ~I_CREATING;
         smp_mb();
         wake_up_bit(&inode->i_state, __I_NEW);
         spin_unlock(&inode->i_lock);
 }
 EXPORT_SYMBOL(d_instantiate_new);
 
 struct dentry *d_make_root(struct inode *root_inode)
 {
         struct dentry *res = NULL;
 
         if (root_inode) {
                 res = d_alloc_anon(root_inode->i_sb);
                 if (res)
                         d_instantiate(res, root_inode);
                 else
                         iput(root_inode);
         }
         return res;
 }
 EXPORT_SYMBOL(d_make_root);
 
 static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
 {
         struct super_block *sb;
         struct dentry *new, *res;
 
         if (!inode)
                 return ERR_PTR(-ESTALE);
         if (IS_ERR(inode))
                 return ERR_CAST(inode);
 
         sb = inode->i_sb;
 
         res = d_find_any_alias(inode); /* existing alias? */
         if (res)
                 goto out;
 
         new = d_alloc_anon(sb);
         if (!new) {
                 res = ERR_PTR(-ENOMEM);
                 goto out;
         }
 
         security_d_instantiate(new, inode);
         spin_lock(&inode->i_lock);
         res = __d_find_any_alias(inode); /* recheck under lock */
         if (likely(!res)) { /* still no alias, attach a disconnected dentry */
                 unsigned add_flags = d_flags_for_inode(inode);
 
                 if (disconnected)
                         add_flags |= DCACHE_DISCONNECTED;
 
                 spin_lock(&new->d_lock);
                 __d_set_inode_and_type(new, inode, add_flags);
                 hlist_add_head(&new->d_u.d_alias, &inode->i_dentry);
                 if (!disconnected) {
                         hlist_bl_lock(&sb->s_roots);
                         hlist_bl_add_head(&new->d_hash, &sb->s_roots);
                         hlist_bl_unlock(&sb->s_roots);
                 }
                 spin_unlock(&new->d_lock);
                 spin_unlock(&inode->i_lock);
                 inode = NULL; /* consumed by new->d_inode */
                 res = new;
         } else {
                 spin_unlock(&inode->i_lock);
                 dput(new);
         }
 
  out:
         iput(inode);
         return res;
 }
 
 /**
  * d_obtain_alias - find or allocate a DISCONNECTED dentry for a given inode
  * @inode: inode to allocate the dentry for
  *
  * Obtain a dentry for an inode resulting from NFS filehandle conversion or
  * similar open by handle operations.  The returned dentry may be anonymous,
  * or may have a full name (if the inode was already in the cache).
  *
  * When called on a directory inode, we must ensure that the inode only ever
  * has one dentry.  If a dentry is found, that is returned instead of
  * allocating a new one.
  *
  * On successful return, the reference to the inode has been transferred
  * to the dentry.  In case of an error the reference on the inode is released.
  * To make it easier to use in export operations a %NULL or IS_ERR inode may
  * be passed in and the error will be propagated to the return value,
  * with a %NULL @inode replaced by ERR_PTR(-ESTALE).
  */
 struct dentry *d_obtain_alias(struct inode *inode)
 {
         return __d_obtain_alias(inode, true);
 }
 EXPORT_SYMBOL(d_obtain_alias);
 
 /**
  * d_obtain_root - find or allocate a dentry for a given inode
  * @inode: inode to allocate the dentry for
  *
  * Obtain an IS_ROOT dentry for the root of a filesystem.
  *
  * We must ensure that directory inodes only ever have one dentry.  If a
  * dentry is found, that is returned instead of allocating a new one.
  *
  * On successful return, the reference to the inode has been transferred
  * to the dentry.  In case of an error the reference on the inode is
  * released.  A %NULL or IS_ERR inode may be passed in and will be the
  * error will be propagate to the return value, with a %NULL @inode
  * replaced by ERR_PTR(-ESTALE).
  */
 struct dentry *d_obtain_root(struct inode *inode)
 {
         return __d_obtain_alias(inode, false);
 }
 EXPORT_SYMBOL(d_obtain_root);
 
 /**
  * d_add_ci - lookup or allocate new dentry with case-exact name
  * @inode:  the inode case-insensitive lookup has found
  * @dentry: the negative dentry that was passed to the parent's lookup func
  * @name:   the case-exact name to be associated with the returned dentry
  *
  * This is to avoid filling the dcache with case-insensitive names to the
  * same inode, only the actual correct case is stored in the dcache for
  * case-insensitive filesystems.
  *
  * For a case-insensitive lookup match and if the case-exact dentry
  * already exists in the dcache, use it and return it.
  *
  * If no entry exists with the exact case name, allocate new dentry with
  * the exact case, and return the spliced entry.
  */
 struct dentry *d_add_ci(struct dentry *dentry, struct inode *inode,
                         struct qstr *name)
 {
         struct dentry *found, *res;
 
         /*
          * First check if a dentry matching the name already exists,
          * if not go ahead and create it now.
          */
         found = d_hash_and_lookup(dentry->d_parent, name);
         if (found) {
                 iput(inode);
                 return found;
         }
         if (d_in_lookup(dentry)) {
                 found = d_alloc_parallel(dentry->d_parent, name,
                                         dentry->d_wait);
                 if (IS_ERR(found) || !d_in_lookup(found)) {
                         iput(inode);
                         return found;
                 }
         } else {
                 found = d_alloc(dentry->d_parent, name);
                 if (!found) {
                         iput(inode);
                         return ERR_PTR(-ENOMEM);
                 } 
         }
         res = d_splice_alias(inode, found);
         if (res) {
                 d_lookup_done(found);
                 dput(found);
                 return res;
         }
         return found;
 }
 EXPORT_SYMBOL(d_add_ci);
 
 /**
  * d_same_name - compare dentry name with case-exact name
  * @parent: parent dentry
  * @dentry: the negative dentry that was passed to the parent's lookup func
  * @name:   the case-exact name to be associated with the returned dentry
  *
  * Return: true if names are same, or false
  */
 bool d_same_name(const struct dentry *dentry, const struct dentry *parent,
                  const struct qstr *name)
 {
         if (likely(!(parent->d_flags & DCACHE_OP_COMPARE))) {
                 if (dentry->d_name.len != name->len)
                         return false;
                 return dentry_cmp(dentry, name->name, name->len) == 0;
         }
         return parent->d_op->d_compare(dentry,
                                        dentry->d_name.len, dentry->d_name.name,
                                        name) == 0;
 }
 EXPORT_SYMBOL_GPL(d_same_name);
 
 /*
  * This is __d_lookup_rcu() when the parent dentry has
  * DCACHE_OP_COMPARE, which makes things much nastier.
  */
 static noinline struct dentry *__d_lookup_rcu_op_compare(
         const struct dentry *parent,
         const struct qstr *name,
         unsigned *seqp)
 {
         u64 hashlen = name->hash_len;
         struct hlist_bl_head *b = d_hash(hashlen);
         struct hlist_bl_node *node;
         struct dentry *dentry;
 
         hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
                 int tlen;
                 const char *tname;
                 unsigned seq;
 
 seqretry:
                 seq = raw_seqcount_begin(&dentry->d_seq);
                 if (dentry->d_parent != parent)
                         continue;
                 if (d_unhashed(dentry))
                         continue;
                 if (dentry->d_name.hash != hashlen_hash(hashlen))
                         continue;
                 tlen = dentry->d_name.len;
                 tname = dentry->d_name.name;
                 /* we want a consistent (name,len) pair */
                 if (read_seqcount_retry(&dentry->d_seq, seq)) {
                         cpu_relax();
                         goto seqretry;
                 }
                 if (parent->d_op->d_compare(dentry, tlen, tname, name) != 0)
                         continue;
                 *seqp = seq;
                 return dentry;
         }
         return NULL;
 }
 
 /**
  * __d_lookup_rcu - search for a dentry (racy, store-free)
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * @seqp: returns d_seq value at the point where the dentry was found
  * Returns: dentry, or NULL
  *
  * __d_lookup_rcu is the dcache lookup function for rcu-walk name
  * resolution (store-free path walking) design described in
  * Documentation/filesystems/path-lookup.txt.
  *
  * This is not to be used outside core vfs.
  *
  * __d_lookup_rcu must only be used in rcu-walk mode, ie. with vfsmount lock
  * held, and rcu_read_lock held. The returned dentry must not be stored into
  * without taking d_lock and checking d_seq sequence count against @seq
  * returned here.
  *
  * A refcount may be taken on the found dentry with the d_rcu_to_refcount
  * function.
  *
  * Alternatively, __d_lookup_rcu may be called again to look up the child of
  * the returned dentry, so long as its parent's seqlock is checked after the
  * child is looked up. Thus, an interlocking stepping of sequence lock checks
  * is formed, giving integrity down the path walk.
  *
  * NOTE! The caller *has* to check the resulting dentry against the sequence
  * number we've returned before using any of the resulting dentry state!
  */
 struct dentry *__d_lookup_rcu(const struct dentry *parent,
                                 const struct qstr *name,
                                 unsigned *seqp)
 {
         u64 hashlen = name->hash_len;
         const unsigned char *str = name->name;
         struct hlist_bl_head *b = d_hash(hashlen);
         struct hlist_bl_node *node;
         struct dentry *dentry;
 
         /*
          * Note: There is significant duplication with __d_lookup_rcu which is
          * required to prevent single threaded performance regressions
          * especially on architectures where smp_rmb (in seqcounts) are costly.
          * Keep the two functions in sync.
          */
 
         if (unlikely(parent->d_flags & DCACHE_OP_COMPARE))
                 return __d_lookup_rcu_op_compare(parent, name, seqp);
 
         /*
          * The hash list is protected using RCU.
          *
          * Carefully use d_seq when comparing a candidate dentry, to avoid
          * races with d_move().
          *
          * It is possible that concurrent renames can mess up our list
          * walk here and result in missing our dentry, resulting in the
          * false-negative result. d_lookup() protects against concurrent
          * renames using rename_lock seqlock.
          *
          * See Documentation/filesystems/path-lookup.txt for more details.
          */
         hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
                 unsigned seq;
 
                 /*
                  * The dentry sequence count protects us from concurrent
                  * renames, and thus protects parent and name fields.
                  *
                  * The caller must perform a seqcount check in order
                  * to do anything useful with the returned dentry.
                  *
                  * NOTE! We do a "raw" seqcount_begin here. That means that
                  * we don't wait for the sequence count to stabilize if it
                  * is in the middle of a sequence change. If we do the slow
                  * dentry compare, we will do seqretries until it is stable,
                  * and if we end up with a successful lookup, we actually
                  * want to exit RCU lookup anyway.
                  *
                  * Note that raw_seqcount_begin still *does* smp_rmb(), so
                  * we are still guaranteed NUL-termination of ->d_name.name.
                  */
                 seq = raw_seqcount_begin(&dentry->d_seq);
                 if (dentry->d_parent != parent)
                         continue;
                 if (d_unhashed(dentry))
                         continue;
                 if (dentry->d_name.hash_len != hashlen)
                         continue;
                 if (dentry_cmp(dentry, str, hashlen_len(hashlen)) != 0)
                         continue;
                 *seqp = seq;
                 return dentry;
         }
         return NULL;
 }
 
 /**
  * d_lookup - search for a dentry
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * Returns: dentry, or NULL
  *
  * d_lookup searches the children of the parent dentry for the name in
  * question. If the dentry is found its reference count is incremented and the
  * dentry is returned. The caller must use dput to free the entry when it has
  * finished using it. %NULL is returned if the dentry does not exist.
  */
 struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
 {
         struct dentry *dentry;
         unsigned seq;
 
         do {
                 seq = read_seqbegin(&rename_lock);
                 dentry = __d_lookup(parent, name);
                 if (dentry)
                         break;
         } while (read_seqretry(&rename_lock, seq));
         return dentry;
 }
 EXPORT_SYMBOL(d_lookup);
 
 /**
  * __d_lookup - search for a dentry (racy)
  * @parent: parent dentry
  * @name: qstr of name we wish to find
  * Returns: dentry, or NULL
  *
  * __d_lookup is like d_lookup, however it may (rarely) return a
  * false-negative result due to unrelated rename activity.
  *
  * __d_lookup is slightly faster by avoiding rename_lock read seqlock,
  * however it must be used carefully, eg. with a following d_lookup in
  * the case of failure.
  *
  * __d_lookup callers must be commented.
  */
 struct dentry *__d_lookup(const struct dentry *parent, const struct qstr *name)
 {
         unsigned int hash = name->hash;
         struct hlist_bl_head *b = d_hash(hash);
         struct hlist_bl_node *node;
         struct dentry *found = NULL;
         struct dentry *dentry;
 
         /*
          * Note: There is significant duplication with __d_lookup_rcu which is
          * required to prevent single threaded performance regressions
          * especially on architectures where smp_rmb (in seqcounts) are costly.
          * Keep the two functions in sync.
          */
 
         /*
          * The hash list is protected using RCU.
          *
          * Take d_lock when comparing a candidate dentry, to avoid races
          * with d_move().
          *
          * It is possible that concurrent renames can mess up our list
          * walk here and result in missing our dentry, resulting in the
          * false-negative result. d_lookup() protects against concurrent
          * renames using rename_lock seqlock.
          *
          * See Documentation/filesystems/path-lookup.txt for more details.
          */
         rcu_read_lock();
         
         hlist_bl_for_each_entry_rcu(dentry, node, b, d_hash) {
 
                 if (dentry->d_name.hash != hash)
                         continue;
 
                 spin_lock(&dentry->d_lock);
                 if (dentry->d_parent != parent)
                         goto next;
                 if (d_unhashed(dentry))
                         goto next;
 
                 if (!d_same_name(dentry, parent, name))
                         goto next;
 
                 dentry->d_lockref.count++;
                 found = dentry;
                 spin_unlock(&dentry->d_lock);
                 break;
 next:
                 spin_unlock(&dentry->d_lock);
         }
         rcu_read_unlock();
 
         return found;
 }
 
 /**
  * d_hash_and_lookup - hash the qstr then search for a dentry
  * @dir: Directory to search in
  * @name: qstr of name we wish to find
  *
  * On lookup failure NULL is returned; on bad name - ERR_PTR(-error)
  */
 struct dentry *d_hash_and_lookup(struct dentry *dir, struct qstr *name)
 {
         /*
          * Check for a fs-specific hash function. Note that we must
          * calculate the standard hash first, as the d_op->d_hash()
          * routine may choose to leave the hash value unchanged.
          */
         name->hash = full_name_hash(dir, name->name, name->len);
         if (dir->d_flags & DCACHE_OP_HASH) {
                 int err = dir->d_op->d_hash(dir, name);
                 if (unlikely(err < 0))
                         return ERR_PTR(err);
         }
         return d_lookup(dir, name);
 }
 EXPORT_SYMBOL(d_hash_and_lookup);
 
 /*
  * When a file is deleted, we have two options:
  * - turn this dentry into a negative dentry
  * - unhash this dentry and free it.
  *
  * Usually, we want to just turn this into
  * a negative dentry, but if anybody else is
  * currently using the dentry or the inode
  * we can't do that and we fall back on removing
  * it from the hash queues and waiting for
  * it to be deleted later when it has no users
  */
  
 /**
  * d_delete - delete a dentry
  * @dentry: The dentry to delete
  *
  * Turn the dentry into a negative dentry if possible, otherwise
  * remove it from the hash queues so it can be deleted later
  */
  
 void d_delete(struct dentry * dentry)
 {
         struct inode *inode = dentry->d_inode;
 
         spin_lock(&inode->i_lock);
         spin_lock(&dentry->d_lock);
         /*
          * Are we the only user?
          */
         if (dentry->d_lockref.count == 1) {
                 dentry->d_flags &= ~DCACHE_CANT_MOUNT;
                 dentry_unlink_inode(dentry);
         } else {
                 __d_drop(dentry);
                 spin_unlock(&dentry->d_lock);
                 spin_unlock(&inode->i_lock);
         }
 }
 EXPORT_SYMBOL(d_delete);
 
 static void __d_rehash(struct dentry *entry)
 {
         struct hlist_bl_head *b = d_hash(entry->d_name.hash);
 
         hlist_bl_lock(b);
         hlist_bl_add_head_rcu(&entry->d_hash, b);
         hlist_bl_unlock(b);
 }
 
 /**
  * d_rehash     - add an entry back to the hash
  * @entry: dentry to add to the hash
  *
  * Adds a dentry to the hash according to its name.
  */
  
 void d_rehash(struct dentry * entry)
 {
         spin_lock(&entry->d_lock);
         __d_rehash(entry);
         spin_unlock(&entry->d_lock);
 }
 EXPORT_SYMBOL(d_rehash);
 
 static inline unsigned start_dir_add(struct inode *dir)
 {
         preempt_disable_nested();
         for (;;) {
                 unsigned n = dir->i_dir_seq;
                 if (!(n & 1) && cmpxchg(&dir->i_dir_seq, n, n + 1) == n)
                         return n;
                 cpu_relax();
         }
 }
 
 static inline void end_dir_add(struct inode *dir, unsigned int n,
                                wait_queue_head_t *d_wait)
 {
         smp_store_release(&dir->i_dir_seq, n + 2);
         preempt_enable_nested();
         wake_up_all(d_wait);
 }
 
 static void d_wait_lookup(struct dentry *dentry)
 {
         if (d_in_lookup(dentry)) {
                 DECLARE_WAITQUEUE(wait, current);
                 add_wait_queue(dentry->d_wait, &wait);
                 do {
                         set_current_state(TASK_UNINTERRUPTIBLE);
                         spin_unlock(&dentry->d_lock);
                         schedule();
                         spin_lock(&dentry->d_lock);
                 } while (d_in_lookup(dentry));
         }
 }
 
 struct dentry *d_alloc_parallel(struct dentry *parent,
                                 const struct qstr *name,
                                 wait_queue_head_t *wq)
 {
         unsigned int hash = name->hash;
         struct hlist_bl_head *b = in_lookup_hash(parent, hash);
         struct hlist_bl_node *node;
         struct dentry *new = d_alloc(parent, name);
         struct dentry *dentry;
         unsigned seq, r_seq, d_seq;
 
         if (unlikely(!new))
                 return ERR_PTR(-ENOMEM);
 
 retry:
         rcu_read_lock();
         seq = smp_load_acquire(&parent->d_inode->i_dir_seq);
         r_seq = read_seqbegin(&rename_lock);
         dentry = __d_lookup_rcu(parent, name, &d_seq);
         if (unlikely(dentry)) {
                 if (!lockref_get_not_dead(&dentry->d_lockref)) {
                         rcu_read_unlock();
                         goto retry;
                 }
                 if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
                         rcu_read_unlock();
                         dput(dentry);
                         goto retry;
                 }
                 rcu_read_unlock();
                 dput(new);
                 return dentry;
         }
         if (unlikely(read_seqretry(&rename_lock, r_seq))) {
                 rcu_read_unlock();
                 goto retry;
         }
 
         if (unlikely(seq & 1)) {
                 rcu_read_unlock();
                 goto retry;
         }
 
         hlist_bl_lock(b);
         if (unlikely(READ_ONCE(parent->d_inode->i_dir_seq) != seq)) {
                 hlist_bl_unlock(b);
                 rcu_read_unlock();
                 goto retry;
         }
         /*
          * No changes for the parent since the beginning of d_lookup().
          * Since all removals from the chain happen with hlist_bl_lock(),
          * any potential in-lookup matches are going to stay here until
          * we unlock the chain.  All fields are stable in everything
          * we encounter.
          */
         hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
                 if (dentry->d_name.hash != hash)
                         continue;
                 if (dentry->d_parent != parent)
                         continue;
                 if (!d_same_name(dentry, parent, name))
                         continue;
                 hlist_bl_unlock(b);
                 /* now we can try to grab a reference */
                 if (!lockref_get_not_dead(&dentry->d_lockref)) {
                         rcu_read_unlock();
                         goto retry;
                 }
 
                 rcu_read_unlock();
                 /*
                  * somebody is likely to be still doing lookup for it;
                  * wait for them to finish
                  */
                 spin_lock(&dentry->d_lock);
                 d_wait_lookup(dentry);
                 /*
                  * it's not in-lookup anymore; in principle we should repeat
                  * everything from dcache lookup, but it's likely to be what
                  * d_lookup() would've found anyway.  If it is, just return it;
                  * otherwise we really have to repeat the whole thing.
                  */
                 if (unlikely(dentry->d_name.hash != hash))
                         goto mismatch;
                 if (unlikely(dentry->d_parent != parent))
                         goto mismatch;
                 if (unlikely(d_unhashed(dentry)))
                         goto mismatch;
                 if (unlikely(!d_same_name(dentry, parent, name)))
                         goto mismatch;
                 /* OK, it *is* a hashed match; return it */
                 spin_unlock(&dentry->d_lock);
                 dput(new);
                 return dentry;
         }
         rcu_read_unlock();
         /* we can't take ->d_lock here; it's OK, though. */
         new->d_flags |= DCACHE_PAR_LOOKUP;
         new->d_wait = wq;
         hlist_bl_add_head(&new->d_u.d_in_lookup_hash, b);
         hlist_bl_unlock(b);
         return new;
 mismatch:
         spin_unlock(&dentry->d_lock);
         dput(dentry);
         goto retry;
 }
 EXPORT_SYMBOL(d_alloc_parallel);
 
 /*
  * - Unhash the dentry
  * - Retrieve and clear the waitqueue head in dentry
  * - Return the waitqueue head
  */
 static wait_queue_head_t *__d_lookup_unhash(struct dentry *dentry)
 {
         wait_queue_head_t *d_wait;
         struct hlist_bl_head *b;
 
         lockdep_assert_held(&dentry->d_lock);
 
         b = in_lookup_hash(dentry->d_parent, dentry->d_name.hash);
         hlist_bl_lock(b);
         dentry->d_flags &= ~DCACHE_PAR_LOOKUP;
         __hlist_bl_del(&dentry->d_u.d_in_lookup_hash);
         d_wait = dentry->d_wait;
         dentry->d_wait = NULL;
         hlist_bl_unlock(b);
         INIT_HLIST_NODE(&dentry->d_u.d_alias);
         INIT_LIST_HEAD(&dentry->d_lru);
         return d_wait;
 }
 
 void __d_lookup_unhash_wake(struct dentry *dentry)
 {
         spin_lock(&dentry->d_lock);
         wake_up_all(__d_lookup_unhash(dentry));
         spin_unlock(&dentry->d_lock);
 }
 EXPORT_SYMBOL(__d_lookup_unhash_wake);
 
 /* inode->i_lock held if inode is non-NULL */
 
 static inline void __d_add(struct dentry *dentry, struct inode *inode)
 {
         wait_queue_head_t *d_wait;
         struct inode *dir = NULL;
         unsigned n;
         spin_lock(&dentry->d_lock);
         if (unlikely(d_in_lookup(dentry))) {
                 dir = dentry->d_parent->d_inode;
                 n = start_dir_add(dir);
                 d_wait = __d_lookup_unhash(dentry);
         }
         if (inode) {
                 unsigned add_flags = d_flags_for_inode(inode);
                 hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
                 raw_write_seqcount_begin(&dentry->d_seq);
                 __d_set_inode_and_type(dentry, inode, add_flags);
                 raw_write_seqcount_end(&dentry->d_seq);
                 fsnotify_update_flags(dentry);
         }
         __d_rehash(dentry);
         if (dir)
                 end_dir_add(dir, n, d_wait);
         spin_unlock(&dentry->d_lock);
         if (inode)
                 spin_unlock(&inode->i_lock);
 }
 
 /**
  * d_add - add dentry to hash queues
  * @entry: dentry to add
  * @inode: The inode to attach to this dentry
  *
  * This adds the entry to the hash queues and initializes @inode.
  * The entry was actually filled in earlier during d_alloc().
  */
 
 void d_add(struct dentry *entry, struct inode *inode)
 {
         if (inode) {
                 security_d_instantiate(entry, inode);
                 spin_lock(&inode->i_lock);
         }
         __d_add(entry, inode);
 }
 EXPORT_SYMBOL(d_add);
 
 /**
  * d_exact_alias - find and hash an exact unhashed alias
  * @entry: dentry to add
  * @inode: The inode to go with this dentry
  *
  * If an unhashed dentry with the same name/parent and desired
  * inode already exists, hash and return it.  Otherwise, return
  * NULL.
  *
  * Parent directory should be locked.
  */
 struct dentry *d_exact_alias(struct dentry *entry, struct inode *inode)
 {
         struct dentry *alias;
         unsigned int hash = entry->d_name.hash;
 
         spin_lock(&inode->i_lock);
         hlist_for_each_entry(alias, &inode->i_dentry, d_u.d_alias) {
                 /*
                  * Don't need alias->d_lock here, because aliases with
                  * d_parent == entry->d_parent are not subject to name or
                  * parent changes, because the parent inode i_mutex is held.
                  */
                 if (alias->d_name.hash != hash)
                         continue;
                 if (alias->d_parent != entry->d_parent)
                         continue;
                 if (!d_same_name(alias, entry->d_parent, &entry->d_name))
                         continue;
                 spin_lock(&alias->d_lock);
                 if (!d_unhashed(alias)) {
                         spin_unlock(&alias->d_lock);
                         alias = NULL;
                 } else {
                         dget_dlock(alias);
                         __d_rehash(alias);
                         spin_unlock(&alias->d_lock);
                 }
                 spin_unlock(&inode->i_lock);
                 return alias;
         }
         spin_unlock(&inode->i_lock);
         return NULL;
 }
 EXPORT_SYMBOL(d_exact_alias);
 
 static void swap_names(struct dentry *dentry, struct dentry *target)
 {
         if (unlikely(dname_external(target))) {
                 if (unlikely(dname_external(dentry))) {
                         /*
                          * Both external: swap the pointers
                          */
                         swap(target->d_name.name, dentry->d_name.name);
                 } else {
                         /*
                          * dentry:internal, target:external.  Steal target's
                          * storage and make target internal.
                          */
                         memcpy(target->d_iname, dentry->d_name.name,
                                         dentry->d_name.len + 1);
                         dentry->d_name.name = target->d_name.name;
                         target->d_name.name = target->d_iname;
                 }
         } else {
                 if (unlikely(dname_external(dentry))) {
                         /*
                          * dentry:external, target:internal.  Give dentry's
                          * storage to target and make dentry internal
                          */
                         memcpy(dentry->d_iname, target->d_name.name,
                                         target->d_name.len + 1);
                         target->d_name.name = dentry->d_name.name;
                         dentry->d_name.name = dentry->d_iname;
                 } else {
                         /*
                          * Both are internal.
                          */
                         unsigned int i;
                         BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
                         for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
                                 swap(((long *) &dentry->d_iname)[i],
                                      ((long *) &target->d_iname)[i]);
                         }
                 }
         }
         swap(dentry->d_name.hash_len, target->d_name.hash_len);
 }
 
 static void copy_name(struct dentry *dentry, struct dentry *target)
 {
         struct external_name *old_name = NULL;
         if (unlikely(dname_external(dentry)))
                 old_name = external_name(dentry);
         if (unlikely(dname_external(target))) {
                 atomic_inc(&external_name(target)->u.count);
                 dentry->d_name = target->d_name;
         } else {
                 memcpy(dentry->d_iname, target->d_name.name,
                                 target->d_name.len + 1);
                 dentry->d_name.name = dentry->d_iname;
                 dentry->d_name.hash_len = target->d_name.hash_len;
         }
         if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
                 kfree_rcu(old_name, u.head);
 }
 
 /*
  * __d_move - move a dentry
  * @dentry: entry to move
  * @target: new dentry
  * @exchange: exchange the two dentries
  *
  * Update the dcache to reflect the move of a file name. Negative
  * dcache entries should not be moved in this way. Caller must hold
  * rename_lock, the i_mutex of the source and target directories,
  * and the sb->s_vfs_rename_mutex if they differ. See lock_rename().
  */
 static void __d_move(struct dentry *dentry, struct dentry *target,
                      bool exchange)
 {
         struct dentry *old_parent, *p;
         wait_queue_head_t *d_wait;
         struct inode *dir = NULL;
         unsigned n;
 
         WARN_ON(!dentry->d_inode);
         if (WARN_ON(dentry == target))
                 return;
 
         BUG_ON(d_ancestor(target, dentry));
         old_parent = dentry->d_parent;
         p = d_ancestor(old_parent, target);
         if (IS_ROOT(dentry)) {
                 BUG_ON(p);
                 spin_lock(&target->d_parent->d_lock);
         } else if (!p) {
                 /* target is not a descendent of dentry->d_parent */
                 spin_lock(&target->d_parent->d_lock);
                 spin_lock_nested(&old_parent->d_lock, DENTRY_D_LOCK_NESTED);
         } else {
                 BUG_ON(p == dentry);
                 spin_lock(&old_parent->d_lock);
                 if (p != target)
                         spin_lock_nested(&target->d_parent->d_lock,
                                         DENTRY_D_LOCK_NESTED);
         }
         spin_lock_nested(&dentry->d_lock, 2);
         spin_lock_nested(&target->d_lock, 3);
 
         if (unlikely(d_in_lookup(target))) {
                 dir = target->d_parent->d_inode;
                 n = start_dir_add(dir);
                 d_wait = __d_lookup_unhash(target);
         }
 
         write_seqcount_begin(&dentry->d_seq);
         write_seqcount_begin_nested(&target->d_seq, DENTRY_D_LOCK_NESTED);
 
         /* unhash both */
         if (!d_unhashed(dentry))
                 ___d_drop(dentry);
         if (!d_unhashed(target))
                 ___d_drop(target);
 
         /* ... and switch them in the tree */
         dentry->d_parent = target->d_parent;
         if (!exchange) {
                 copy_name(dentry, target);
                 target->d_hash.pprev = NULL;
                 dentry->d_parent->d_lockref.count++;
                 if (dentry != old_parent) /* wasn't IS_ROOT */
                         WARN_ON(!--old_parent->d_lockref.count);
         } else {
                 target->d_parent = old_parent;
                 swap_names(dentry, target);
                 if (!hlist_unhashed(&target->d_sib))
                         __hlist_del(&target->d_sib);
                 hlist_add_head(&target->d_sib, &target->d_parent->d_children);
                 __d_rehash(target);
                 fsnotify_update_flags(target);
         }
         if (!hlist_unhashed(&dentry->d_sib))
                 __hlist_del(&dentry->d_sib);
         hlist_add_head(&dentry->d_sib, &dentry->d_parent->d_children);
         __d_rehash(dentry);
         fsnotify_update_flags(dentry);
         fscrypt_handle_d_move(dentry);
 
         write_seqcount_end(&target->d_seq);
         write_seqcount_end(&dentry->d_seq);
 
         if (dir)
                 end_dir_add(dir, n, d_wait);
 
         if (dentry->d_parent != old_parent)
                 spin_unlock(&dentry->d_parent->d_lock);
         if (dentry != old_parent)
                 spin_unlock(&old_parent->d_lock);
         spin_unlock(&target->d_lock);
         spin_unlock(&dentry->d_lock);
 }
 
 /*
  * d_move - move a dentry
  * @dentry: entry to move
  * @target: new dentry
  *
  * Update the dcache to reflect the move of a file name. Negative
  * dcache entries should not be moved in this way. See the locking
  * requirements for __d_move.
  */
 void d_move(struct dentry *dentry, struct dentry *target)
 {
         write_seqlock(&rename_lock);
         __d_move(dentry, target, false);
         write_sequnlock(&rename_lock);
 }
 EXPORT_SYMBOL(d_move);
 
 /*
  * d_exchange - exchange two dentries
  * @dentry1: first dentry
  * @dentry2: second dentry
  */
 void d_exchange(struct dentry *dentry1, struct dentry *dentry2)
 {
         write_seqlock(&rename_lock);
 
         WARN_ON(!dentry1->d_inode);
         WARN_ON(!dentry2->d_inode);
         WARN_ON(IS_ROOT(dentry1));
         WARN_ON(IS_ROOT(dentry2));
 
         __d_move(dentry1, dentry2, true);
 
         write_sequnlock(&rename_lock);
 }
 
 /**
  * d_ancestor - search for an ancestor
  * @p1: ancestor dentry
  * @p2: child dentry
  *
  * Returns the ancestor dentry of p2 which is a child of p1, if p1 is
  * an ancestor of p2, else NULL.
  */
 struct dentry *d_ancestor(struct dentry *p1, struct dentry *p2)
 {
         struct dentry *p;
 
         for (p = p2; !IS_ROOT(p); p = p->d_parent) {
                 if (p->d_parent == p1)
                         return p;
         }
         return NULL;
 }
 
 /*
  * This helper attempts to cope with remotely renamed directories
  *
  * It assumes that the caller is already holding
  * dentry->d_parent->d_inode->i_mutex, and rename_lock
  *
  * Note: If ever the locking in lock_rename() changes, then please
  * remember to update this too...
  */
 static int __d_unalias(struct dentry *dentry, struct dentry *alias)
 {
         struct mutex *m1 = NULL;
         struct rw_semaphore *m2 = NULL;
         int ret = -ESTALE;
 
         /* If alias and dentry share a parent, then no extra locks required */
         if (alias->d_parent == dentry->d_parent)
                 goto out_unalias;
 
         /* See lock_rename() */
         if (!mutex_trylock(&dentry->d_sb->s_vfs_rename_mutex))
                 goto out_err;
         m1 = &dentry->d_sb->s_vfs_rename_mutex;
         if (!inode_trylock_shared(alias->d_parent->d_inode))
                 goto out_err;
         m2 = &alias->d_parent->d_inode->i_rwsem;
 out_unalias:
         __d_move(alias, dentry, false);
         ret = 0;
 out_err:
         if (m2)
                 up_read(m2);
         if (m1)
                 mutex_unlock(m1);
         return ret;
 }
 
 /**
  * d_splice_alias - splice a disconnected dentry into the tree if one exists
  * @inode:  the inode which may have a disconnected dentry
  * @dentry: a negative dentry which we want to point to the inode.
  *
  * If inode is a directory and has an IS_ROOT alias, then d_move that in
  * place of the given dentry and return it, else simply d_add the inode
  * to the dentry and return NULL.
  *
  * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
  * we should error out: directories can't have multiple aliases.
  *
  * This is needed in the lookup routine of any filesystem that is exportable
  * (via knfsd) so that we can build dcache paths to directories effectively.
  *
  * If a dentry was found and moved, then it is returned.  Otherwise NULL
  * is returned.  This matches the expected return value of ->lookup.
  *
  * Cluster filesystems may call this function with a negative, hashed dentry.
  * In that case, we know that the inode will be a regular file, and also this
  * will only occur during atomic_open. So we need to check for the dentry
  * being already hashed only in the final case.
  */
 struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 {
         if (IS_ERR(inode))
                 return ERR_CAST(inode);
 
         BUG_ON(!d_unhashed(dentry));
 
         if (!inode)
                 goto out;
 
         security_d_instantiate(dentry, inode);
         spin_lock(&inode->i_lock);
         if (S_ISDIR(inode->i_mode)) {
                 struct dentry *new = __d_find_any_alias(inode);
                 if (unlikely(new)) {
                         /* The reference to new ensures it remains an alias */
                         spin_unlock(&inode->i_lock);
                         write_seqlock(&rename_lock);
                         if (unlikely(d_ancestor(new, dentry))) {
                                 write_sequnlock(&rename_lock);
                                 dput(new);
                                 new = ERR_PTR(-ELOOP);
                                 pr_warn_ratelimited(
                                         "VFS: Lookup of '%s' in %s %s"
                                         " would have caused loop\n",
                                         dentry->d_name.name,
                                         inode->i_sb->s_type->name,
                                         inode->i_sb->s_id);
                         } else if (!IS_ROOT(new)) {
                                 struct dentry *old_parent = dget(new->d_parent);
                                 int err = __d_unalias(dentry, new);
                                 write_sequnlock(&rename_lock);
                                 if (err) {
                                         dput(new);
                                         new = ERR_PTR(err);
                                 }
                                 dput(old_parent);
                         } else {
                                 __d_move(new, dentry, false);
                                 write_sequnlock(&rename_lock);
                         }
                         iput(inode);
                         return new;
                 }
         }
 out:
         __d_add(dentry, inode);
         return NULL;
 }
 EXPORT_SYMBOL(d_splice_alias);
 
 /*
  * Test whether new_dentry is a subdirectory of old_dentry.
  *
  * Trivially implemented using the dcache structure
  */
 
 /**
  * is_subdir - is new dentry a subdirectory of old_dentry
  * @new_dentry: new dentry
  * @old_dentry: old dentry
  *
  * Returns true if new_dentry is a subdirectory of the parent (at any depth).
  * Returns false otherwise.
  * Caller must ensure that "new_dentry" is pinned before calling is_subdir()
  */
   
 bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
 {
         bool subdir;
         unsigned seq;
 
         if (new_dentry == old_dentry)
                 return true;
 
         /* Access d_parent under rcu as d_move() may change it. */
         rcu_read_lock();
         seq = read_seqbegin(&rename_lock);
         subdir = d_ancestor(old_dentry, new_dentry);
          /* Try lockless once... */
         if (read_seqretry(&rename_lock, seq)) {
                 /* ...else acquire lock for progress even on deep chains. */
                 read_seqlock_excl(&rename_lock);
                 subdir = d_ancestor(old_dentry, new_dentry);
                 read_sequnlock_excl(&rename_lock);
         }
         rcu_read_unlock();
         return subdir;
 }
 EXPORT_SYMBOL(is_subdir);
 
 static enum d_walk_ret d_genocide_kill(void *data, struct dentry *dentry)
 {
         struct dentry *root = data;
         if (dentry != root) {
                 if (d_unhashed(dentry) || !dentry->d_inode)
                         return D_WALK_SKIP;
 
                 if (!(dentry->d_flags & DCACHE_GENOCIDE)) {
                         dentry->d_flags |= DCACHE_GENOCIDE;
                         dentry->d_lockref.count--;
                 }
         }
         return D_WALK_CONTINUE;
 }
 
 void d_genocide(struct dentry *parent)
 {
         d_walk(parent, parent, d_genocide_kill);
 }
 
 void d_mark_tmpfile(struct file *file, struct inode *inode)
 {
         struct dentry *dentry = file->f_path.dentry;
 
         BUG_ON(dentry->d_name.name != dentry->d_iname ||
                 !hlist_unhashed(&dentry->d_u.d_alias) ||
                 !d_unlinked(dentry));
         spin_lock(&dentry->d_parent->d_lock);
         spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
         dentry->d_name.len = sprintf(dentry->d_iname, "#%llu",
                                 (unsigned long long)inode->i_ino);
         spin_unlock(&dentry->d_lock);
         spin_unlock(&dentry->d_parent->d_lock);
 }
 EXPORT_SYMBOL(d_mark_tmpfile);
 
 void d_tmpfile(struct file *file, struct inode *inode)
 {
         struct dentry *dentry = file->f_path.dentry;
 
         inode_dec_link_count(inode);
         d_mark_tmpfile(file, inode);
         d_instantiate(dentry, inode);
 }
 EXPORT_SYMBOL(d_tmpfile);
 
 /*
  * Obtain inode number of the parent dentry.
  */
 ino_t d_parent_ino(struct dentry *dentry)
 {
         struct dentry *parent;
         struct inode *iparent;
         unsigned seq;
         ino_t ret;
 
         scoped_guard(rcu) {
                 seq = raw_seqcount_begin(&dentry->d_seq);
                 parent = READ_ONCE(dentry->d_parent);
                 iparent = d_inode_rcu(parent);
                 if (likely(iparent)) {
                         ret = iparent->i_ino;
                         if (!read_seqcount_retry(&dentry->d_seq, seq))
                                 return ret;
                 }
         }
 
         spin_lock(&dentry->d_lock);
         ret = dentry->d_parent->d_inode->i_ino;
         spin_unlock(&dentry->d_lock);
         return ret;
 }
 EXPORT_SYMBOL(d_parent_ino);
 
 static __initdata unsigned long dhash_entries;
 static int __init set_dhash_entries(char *str)
 {
         if (!str)
                 return 0;
         dhash_entries = simple_strtoul(str, &str, 0);
         return 1;
 }
 __setup("dhash_entries=", set_dhash_entries);
 
 static void __init dcache_init_early(void)
 {
         /* If hashes are distributed across NUMA nodes, defer
          * hash allocation until vmalloc space is available.
          */
         if (hashdist)
                 return;
 
         dentry_hashtable =
                 alloc_large_system_hash("Dentry cache",
                                         sizeof(struct hlist_bl_head),
                                         dhash_entries,
                                         13,
                                         HASH_EARLY | HASH_ZERO,
                                         &d_hash_shift,
                                         NULL,
                                         0,
                                         0);
         d_hash_shift = 32 - d_hash_shift;
 
         runtime_const_init(shift, d_hash_shift);
         runtime_const_init(ptr, dentry_hashtable);
 }
 
 static void __init dcache_init(void)
 {
         /*
          * A constructor could be added for stable state like the lists,
          * but it is probably not worth it because of the cache nature
          * of the dcache.
          */
         dentry_cache = KMEM_CACHE_USERCOPY(dentry,
                 SLAB_RECLAIM_ACCOUNT|SLAB_PANIC|SLAB_ACCOUNT,
                 d_iname);
 
         /* Hash may have been set up in dcache_init_early */
         if (!hashdist)
                 return;
 
         dentry_hashtable =
                 alloc_large_system_hash("Dentry cache",
                                         sizeof(struct hlist_bl_head),
                                         dhash_entries,
                                         13,
                                         HASH_ZERO,
                                         &d_hash_shift,
                                         NULL,
                                         0,
                                         0);
         d_hash_shift = 32 - d_hash_shift;
 
         runtime_const_init(shift, d_hash_shift);
         runtime_const_init(ptr, dentry_hashtable);
 }
 
 /* SLAB cache for __getname() consumers */
 struct kmem_cache *names_cachep __ro_after_init;
 EXPORT_SYMBOL(names_cachep);
 
 void __init vfs_caches_init_early(void)
 {
         int i;
 
         for (i = 0; i < ARRAY_SIZE(in_lookup_hashtable); i++)
                 INIT_HLIST_BL_HEAD(&in_lookup_hashtable[i]);
 
         dcache_init_early();
         inode_init_early();
 }
 
 void __init vfs_caches_init(void)
 {
         names_cachep = kmem_cache_create_usercopy("names_cache", PATH_MAX, 0,
                         SLAB_HWCACHE_ALIGN|SLAB_PANIC, 0, PATH_MAX, NULL);
 
         dcache_init();
         inode_init();
         files_init();
         files_maxfiles_init();
         mnt_init();
         bdev_cache_init();
         chrdev_init();
 }