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2 A description of what robust futexes are 2 A description of what robust futexes are
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4 4
5 :Started by: Ingo Molnar <mingo@redhat.com> 5 :Started by: Ingo Molnar <mingo@redhat.com>
6 6
7 Background 7 Background
8 ---------- 8 ----------
9 9
10 what are robust futexes? To answer that, we fi 10 what are robust futexes? To answer that, we first need to understand
11 what futexes are: normal futexes are special t 11 what futexes are: normal futexes are special types of locks that in the
12 noncontended case can be acquired/released fro 12 noncontended case can be acquired/released from userspace without having
13 to enter the kernel. 13 to enter the kernel.
14 14
15 A futex is in essence a user-space address, e. 15 A futex is in essence a user-space address, e.g. a 32-bit lock variable
16 field. If userspace notices contention (the lo 16 field. If userspace notices contention (the lock is already owned and
17 someone else wants to grab it too) then the lo 17 someone else wants to grab it too) then the lock is marked with a value
18 that says "there's a waiter pending", and the 18 that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
19 syscall is used to wait for the other guy to r 19 syscall is used to wait for the other guy to release it. The kernel
20 creates a 'futex queue' internally, so that it 20 creates a 'futex queue' internally, so that it can later on match up the
21 waiter with the waker - without them having to 21 waiter with the waker - without them having to know about each other.
22 When the owner thread releases the futex, it n 22 When the owner thread releases the futex, it notices (via the variable
23 value) that there were waiter(s) pending, and 23 value) that there were waiter(s) pending, and does the
24 sys_futex(FUTEX_WAKE) syscall to wake them up. 24 sys_futex(FUTEX_WAKE) syscall to wake them up. Once all waiters have
25 taken and released the lock, the futex is agai 25 taken and released the lock, the futex is again back to 'uncontended'
26 state, and there's no in-kernel state associat 26 state, and there's no in-kernel state associated with it. The kernel
27 completely forgets that there ever was a futex 27 completely forgets that there ever was a futex at that address. This
28 method makes futexes very lightweight and scal 28 method makes futexes very lightweight and scalable.
29 29
30 "Robustness" is about dealing with crashes whi 30 "Robustness" is about dealing with crashes while holding a lock: if a
31 process exits prematurely while holding a pthr 31 process exits prematurely while holding a pthread_mutex_t lock that is
32 also shared with some other process (e.g. yum 32 also shared with some other process (e.g. yum segfaults while holding a
33 pthread_mutex_t, or yum is kill -9-ed), then w 33 pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
34 to be notified that the last owner of the lock 34 to be notified that the last owner of the lock exited in some irregular
35 way. 35 way.
36 36
37 To solve such types of problems, "robust mutex 37 To solve such types of problems, "robust mutex" userspace APIs were
38 created: pthread_mutex_lock() returns an error 38 created: pthread_mutex_lock() returns an error value if the owner exits
39 prematurely - and the new owner can decide whe 39 prematurely - and the new owner can decide whether the data protected by
40 the lock can be recovered safely. 40 the lock can be recovered safely.
41 41
42 There is a big conceptual problem with futex b 42 There is a big conceptual problem with futex based mutexes though: it is
43 the kernel that destroys the owner task (e.g. 43 the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
44 the kernel cannot help with the cleanup: if th 44 the kernel cannot help with the cleanup: if there is no 'futex queue'
45 (and in most cases there is none, futexes bein 45 (and in most cases there is none, futexes being fast lightweight locks)
46 then the kernel has no information to clean up 46 then the kernel has no information to clean up after the held lock!
47 Userspace has no chance to clean up after the 47 Userspace has no chance to clean up after the lock either - userspace is
48 the one that crashes, so it has no opportunity 48 the one that crashes, so it has no opportunity to clean up. Catch-22.
49 49
50 In practice, when e.g. yum is kill -9-ed (or s 50 In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
51 is needed to release that futex based lock. Th 51 is needed to release that futex based lock. This is one of the leading
52 bugreports against yum. 52 bugreports against yum.
53 53
54 To solve this problem, the traditional approac 54 To solve this problem, the traditional approach was to extend the vma
55 (virtual memory area descriptor) concept to ha 55 (virtual memory area descriptor) concept to have a notion of 'pending
56 robust futexes attached to this area'. This ap 56 robust futexes attached to this area'. This approach requires 3 new
57 syscall variants to sys_futex(): FUTEX_REGISTE 57 syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
58 FUTEX_RECOVER. At do_exit() time, all vmas are 58 FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
59 they have a robust_head set. This approach has 59 they have a robust_head set. This approach has two fundamental problems
60 left: 60 left:
61 61
62 - it has quite complex locking and race scena 62 - it has quite complex locking and race scenarios. The vma-based
63 approach had been pending for years, but th 63 approach had been pending for years, but they are still not completely
64 reliable. 64 reliable.
65 65
66 - they have to scan _every_ vma at sys_exit() 66 - they have to scan _every_ vma at sys_exit() time, per thread!
67 67
68 The second disadvantage is a real killer: pthr 68 The second disadvantage is a real killer: pthread_exit() takes around 1
69 microsecond on Linux, but with thousands (or t 69 microsecond on Linux, but with thousands (or tens of thousands) of vmas
70 every pthread_exit() takes a millisecond or mo 70 every pthread_exit() takes a millisecond or more, also totally
71 destroying the CPU's L1 and L2 caches! 71 destroying the CPU's L1 and L2 caches!
72 72
73 This is very much noticeable even for normal p 73 This is very much noticeable even for normal process sys_exit_group()
74 calls: the kernel has to do the vma scanning u 74 calls: the kernel has to do the vma scanning unconditionally! (this is
75 because the kernel has no knowledge about how 75 because the kernel has no knowledge about how many robust futexes there
76 are to be cleaned up, because a robust futex m 76 are to be cleaned up, because a robust futex might have been registered
77 in another task, and the futex variable might 77 in another task, and the futex variable might have been simply mmap()-ed
78 into this process's address space). 78 into this process's address space).
79 79
80 This huge overhead forced the creation of CONF 80 This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
81 normal kernels can turn it off, but worse than 81 normal kernels can turn it off, but worse than that: the overhead makes
82 robust futexes impractical for any type of gen 82 robust futexes impractical for any type of generic Linux distribution.
83 83
84 So something had to be done. 84 So something had to be done.
85 85
86 New approach to robust futexes 86 New approach to robust futexes
87 ------------------------------ 87 ------------------------------
88 88
89 At the heart of this new approach there is a p 89 At the heart of this new approach there is a per-thread private list of
90 robust locks that userspace is holding (mainta 90 robust locks that userspace is holding (maintained by glibc) - which
91 userspace list is registered with the kernel v 91 userspace list is registered with the kernel via a new syscall [this
92 registration happens at most once per thread l 92 registration happens at most once per thread lifetime]. At do_exit()
93 time, the kernel checks this user-space list: 93 time, the kernel checks this user-space list: are there any robust futex
94 locks to be cleaned up? 94 locks to be cleaned up?
95 95
96 In the common case, at do_exit() time, there i 96 In the common case, at do_exit() time, there is no list registered, so
97 the cost of robust futexes is just a simple cu 97 the cost of robust futexes is just a simple current->robust_list != NULL
98 comparison. If the thread has registered a lis 98 comparison. If the thread has registered a list, then normally the list
99 is empty. If the thread/process crashed or ter 99 is empty. If the thread/process crashed or terminated in some incorrect
100 way then the list might be non-empty: in this 100 way then the list might be non-empty: in this case the kernel carefully
101 walks the list [not trusting it], and marks al 101 walks the list [not trusting it], and marks all locks that are owned by
102 this thread with the FUTEX_OWNER_DIED bit, and 102 this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
103 any). 103 any).
104 104
105 The list is guaranteed to be private and per-t 105 The list is guaranteed to be private and per-thread at do_exit() time,
106 so it can be accessed by the kernel in a lockl 106 so it can be accessed by the kernel in a lockless way.
107 107
108 There is one race possible though: since addin 108 There is one race possible though: since adding to and removing from the
109 list is done after the futex is acquired by gl 109 list is done after the futex is acquired by glibc, there is a few
110 instructions window for the thread (or process 110 instructions window for the thread (or process) to die there, leaving
111 the futex hung. To protect against this possib 111 the futex hung. To protect against this possibility, userspace (glibc)
112 also maintains a simple per-thread 'list_op_pe 112 also maintains a simple per-thread 'list_op_pending' field, to allow the
113 kernel to clean up if the thread dies after ac 113 kernel to clean up if the thread dies after acquiring the lock, but just
114 before it could have added itself to the list. 114 before it could have added itself to the list. Glibc sets this
115 list_op_pending field before it tries to acqui 115 list_op_pending field before it tries to acquire the futex, and clears
116 it after the list-add (or list-remove) has fin 116 it after the list-add (or list-remove) has finished.
117 117
118 That's all that is needed - all the rest of ro 118 That's all that is needed - all the rest of robust-futex cleanup is done
119 in userspace [just like with the previous patc 119 in userspace [just like with the previous patches].
120 120
121 Ulrich Drepper has implemented the necessary g 121 Ulrich Drepper has implemented the necessary glibc support for this new
122 mechanism, which fully enables robust mutexes. 122 mechanism, which fully enables robust mutexes.
123 123
124 Key differences of this userspace-list based a 124 Key differences of this userspace-list based approach, compared to the
125 vma based method: 125 vma based method:
126 126
127 - it's much, much faster: at thread exit time 127 - it's much, much faster: at thread exit time, there's no need to loop
128 over every vma (!), which the VM-based meth 128 over every vma (!), which the VM-based method has to do. Only a very
129 simple 'is the list empty' op is done. 129 simple 'is the list empty' op is done.
130 130
131 - no VM changes are needed - 'struct address_ 131 - no VM changes are needed - 'struct address_space' is left alone.
132 132
133 - no registration of individual locks is need 133 - no registration of individual locks is needed: robust mutexes don't
134 need any extra per-lock syscalls. Robust mu 134 need any extra per-lock syscalls. Robust mutexes thus become a very
135 lightweight primitive - so they don't force 135 lightweight primitive - so they don't force the application designer
136 to do a hard choice between performance and 136 to do a hard choice between performance and robustness - robust
137 mutexes are just as fast. 137 mutexes are just as fast.
138 138
139 - no per-lock kernel allocation happens. 139 - no per-lock kernel allocation happens.
140 140
141 - no resource limits are needed. 141 - no resource limits are needed.
142 142
143 - no kernel-space recovery call (FUTEX_RECOVE 143 - no kernel-space recovery call (FUTEX_RECOVER) is needed.
144 144
145 - the implementation and the locking is "obvi 145 - the implementation and the locking is "obvious", and there are no
146 interactions with the VM. 146 interactions with the VM.
147 147
148 Performance 148 Performance
149 ----------- 149 -----------
150 150
151 I have benchmarked the time needed for the ker 151 I have benchmarked the time needed for the kernel to process a list of 1
152 million (!) held locks, using the new method [ 152 million (!) held locks, using the new method [on a 2GHz CPU]:
153 153
154 - with FUTEX_WAIT set [contended mutex]: 130 154 - with FUTEX_WAIT set [contended mutex]: 130 msecs
155 - without FUTEX_WAIT set [uncontended mutex]: 155 - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
156 156
157 I have also measured an approach where glibc d 157 I have also measured an approach where glibc does the lock notification
158 [which it currently does for !pshared robust m 158 [which it currently does for !pshared robust mutexes], and that took 256
159 msecs - clearly slower, due to the 1 million F 159 msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
160 userspace had to do. 160 userspace had to do.
161 161
162 (1 million held locks are unheard of - we expe 162 (1 million held locks are unheard of - we expect at most a handful of
163 locks to be held at a time. Nevertheless it's 163 locks to be held at a time. Nevertheless it's nice to know that this
164 approach scales nicely.) 164 approach scales nicely.)
165 165
166 Implementation details 166 Implementation details
167 ---------------------- 167 ----------------------
168 168
169 The patch adds two new syscalls: one to regist 169 The patch adds two new syscalls: one to register the userspace list, and
170 one to query the registered list pointer:: 170 one to query the registered list pointer::
171 171
172 asmlinkage long 172 asmlinkage long
173 sys_set_robust_list(struct robust_list_head _ 173 sys_set_robust_list(struct robust_list_head __user *head,
174 size_t len); 174 size_t len);
175 175
176 asmlinkage long 176 asmlinkage long
177 sys_get_robust_list(int pid, struct robust_li 177 sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
178 size_t __user *len_ptr); 178 size_t __user *len_ptr);
179 179
180 List registration is very fast: the pointer is 180 List registration is very fast: the pointer is simply stored in
181 current->robust_list. [Note that in the future 181 current->robust_list. [Note that in the future, if robust futexes become
182 widespread, we could extend sys_clone() to reg 182 widespread, we could extend sys_clone() to register a robust-list head
183 for new threads, without the need of another s 183 for new threads, without the need of another syscall.]
184 184
185 So there is virtually zero overhead for tasks 185 So there is virtually zero overhead for tasks not using robust futexes,
186 and even for robust futex users, there is only 186 and even for robust futex users, there is only one extra syscall per
187 thread lifetime, and the cleanup operation, if 187 thread lifetime, and the cleanup operation, if it happens, is fast and
188 straightforward. The kernel doesn't have any i 188 straightforward. The kernel doesn't have any internal distinction between
189 robust and normal futexes. 189 robust and normal futexes.
190 190
191 If a futex is found to be held at exit time, t 191 If a futex is found to be held at exit time, the kernel sets the
192 following bit of the futex word:: 192 following bit of the futex word::
193 193
194 #define FUTEX_OWNER_DIED 0x4000 194 #define FUTEX_OWNER_DIED 0x40000000
195 195
196 and wakes up the next futex waiter (if any). U 196 and wakes up the next futex waiter (if any). User-space does the rest of
197 the cleanup. 197 the cleanup.
198 198
199 Otherwise, robust futexes are acquired by glib 199 Otherwise, robust futexes are acquired by glibc by putting the TID into
200 the futex field atomically. Waiters set the FU 200 the futex field atomically. Waiters set the FUTEX_WAITERS bit::
201 201
202 #define FUTEX_WAITERS 0x8000 202 #define FUTEX_WAITERS 0x80000000
203 203
204 and the remaining bits are for the TID. 204 and the remaining bits are for the TID.
205 205
206 Testing, architecture support 206 Testing, architecture support
207 ----------------------------- 207 -----------------------------
208 208
209 I've tested the new syscalls on x86 and x86_64 209 I've tested the new syscalls on x86 and x86_64, and have made sure the
210 parsing of the userspace list is robust [ ;-) 210 parsing of the userspace list is robust [ ;-) ] even if the list is
211 deliberately corrupted. 211 deliberately corrupted.
212 212
213 i386 and x86_64 syscalls are wired up at the m 213 i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
214 tested the new glibc code (on x86_64 and i386) 214 tested the new glibc code (on x86_64 and i386), and it works for his
215 robust-mutex testcases. 215 robust-mutex testcases.
216 216
217 All other architectures should build just fine 217 All other architectures should build just fine too - but they won't have
218 the new syscalls yet. 218 the new syscalls yet.
219 219
220 Architectures need to implement the new futex_ 220 Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
221 inline function before writing up the syscalls 221 inline function before writing up the syscalls.
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