1 ===============
2 Locking lessons
3 ===============
4
5 Lesson 1: Spin locks
6 ====================
7
8 The most basic primitive for locking is spinlock::
9
10 static DEFINE_SPINLOCK(xxx_lock);
11
12 unsigned long flags;
13
14 spin_lock_irqsave(&xxx_lock, flags);
15 ... critical section here ..
16 spin_unlock_irqrestore(&xxx_lock, flags);
17
18 The above is always safe. It will disable interrupts _locally_, but the
19 spinlock itself will guarantee the global lock, so it will guarantee that
20 there is only one thread-of-control within the region(s) protected by that
21 lock. This works well even under UP also, so the code does _not_ need to
22 worry about UP vs SMP issues: the spinlocks work correctly under both.
23
24 NOTE! Implications of spin_locks for memory are further described in:
25
26 Documentation/memory-barriers.txt
27
28 (5) ACQUIRE operations.
29
30 (6) RELEASE operations.
31
32 The above is usually pretty simple (you usually need and want only one
33 spinlock for most things - using more than one spinlock can make things a
34 lot more complex and even slower and is usually worth it only for
35 sequences that you **know** need to be split up: avoid it at all cost if you
36 aren't sure).
37
38 This is really the only really hard part about spinlocks: once you start
39 using spinlocks they tend to expand to areas you might not have noticed
40 before, because you have to make sure the spinlocks correctly protect the
41 shared data structures **everywhere** they are used. The spinlocks are most
42 easily added to places that are completely independent of other code (for
43 example, internal driver data structures that nobody else ever touches).
44
45 NOTE! The spin-lock is safe only when you **also** use the lock itself
46 to do locking across CPU's, which implies that EVERYTHING that
47 touches a shared variable has to agree about the spinlock they want
48 to use.
49
50 ----
51
52 Lesson 2: reader-writer spinlocks.
53 ==================================
54
55 If your data accesses have a very natural pattern where you usually tend
56 to mostly read from the shared variables, the reader-writer locks
57 (rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
58 readers to be in the same critical region at once, but if somebody wants
59 to change the variables it has to get an exclusive write lock.
60
61 NOTE! reader-writer locks require more atomic memory operations than
62 simple spinlocks. Unless the reader critical section is long, you
63 are better off just using spinlocks.
64
65 The routines look the same as above::
66
67 rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
68
69 unsigned long flags;
70
71 read_lock_irqsave(&xxx_lock, flags);
72 .. critical section that only reads the info ...
73 read_unlock_irqrestore(&xxx_lock, flags);
74
75 write_lock_irqsave(&xxx_lock, flags);
76 .. read and write exclusive access to the info ...
77 write_unlock_irqrestore(&xxx_lock, flags);
78
79 The above kind of lock may be useful for complex data structures like
80 linked lists, especially searching for entries without changing the list
81 itself. The read lock allows many concurrent readers. Anything that
82 **changes** the list will have to get the write lock.
83
84 NOTE! RCU is better for list traversal, but requires careful
85 attention to design detail (see Documentation/RCU/listRCU.rst).
86
87 Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
88 time need to do any changes (even if you don't do it every time), you have
89 to get the write-lock at the very beginning.
90
91 NOTE! We are working hard to remove reader-writer spinlocks in most
92 cases, so please don't add a new one without consensus. (Instead, see
93 Documentation/RCU/rcu.rst for complete information.)
94
95 ----
96
97 Lesson 3: spinlocks revisited.
98 ==============================
99
100 The single spin-lock primitives above are by no means the only ones. They
101 are the most safe ones, and the ones that work under all circumstances,
102 but partly **because** they are safe they are also fairly slow. They are slower
103 than they'd need to be, because they do have to disable interrupts
104 (which is just a single instruction on a x86, but it's an expensive one -
105 and on other architectures it can be worse).
106
107 If you have a case where you have to protect a data structure across
108 several CPU's and you want to use spinlocks you can potentially use
109 cheaper versions of the spinlocks. IFF you know that the spinlocks are
110 never used in interrupt handlers, you can use the non-irq versions::
111
112 spin_lock(&lock);
113 ...
114 spin_unlock(&lock);
115
116 (and the equivalent read-write versions too, of course). The spinlock will
117 guarantee the same kind of exclusive access, and it will be much faster.
118 This is useful if you know that the data in question is only ever
119 manipulated from a "process context", ie no interrupts involved.
120
121 The reasons you mustn't use these versions if you have interrupts that
122 play with the spinlock is that you can get deadlocks::
123
124 spin_lock(&lock);
125 ...
126 <- interrupt comes in:
127 spin_lock(&lock);
128
129 where an interrupt tries to lock an already locked variable. This is ok if
130 the other interrupt happens on another CPU, but it is _not_ ok if the
131 interrupt happens on the same CPU that already holds the lock, because the
132 lock will obviously never be released (because the interrupt is waiting
133 for the lock, and the lock-holder is interrupted by the interrupt and will
134 not continue until the interrupt has been processed).
135
136 (This is also the reason why the irq-versions of the spinlocks only need
137 to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
138 on other CPU's, because an interrupt on another CPU doesn't interrupt the
139 CPU that holds the lock, so the lock-holder can continue and eventually
140 releases the lock).
141
142 Linus
143
144 ----
145
146 Reference information:
147 ======================
148
149 For dynamic initialization, use spin_lock_init() or rwlock_init() as
150 appropriate::
151
152 spinlock_t xxx_lock;
153 rwlock_t xxx_rw_lock;
154
155 static int __init xxx_init(void)
156 {
157 spin_lock_init(&xxx_lock);
158 rwlock_init(&xxx_rw_lock);
159 ...
160 }
161
162 module_init(xxx_init);
163
164 For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
165 __SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
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