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William Allen Simpsonfb0bbb92009-12-13 15:12:46 -05001Lesson 1: Spin locks
Linus Torvalds1da177e2005-04-16 15:20:36 -07002
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -05003The most basic primitive for locking is spinlock.
Ed L. Cashin017f0212007-07-15 23:41:50 -07004
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -05005static DEFINE_SPINLOCK(xxx_lock);
Linus Torvalds1da177e2005-04-16 15:20:36 -07006
7 unsigned long flags;
8
9 spin_lock_irqsave(&xxx_lock, flags);
10 ... critical section here ..
11 spin_unlock_irqrestore(&xxx_lock, flags);
12
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050013The above is always safe. It will disable interrupts _locally_, but the
Linus Torvalds1da177e2005-04-16 15:20:36 -070014spinlock itself will guarantee the global lock, so it will guarantee that
15there is only one thread-of-control within the region(s) protected by that
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050016lock. This works well even under UP. The above sequence under UP
17essentially is just the same as doing
Linus Torvalds1da177e2005-04-16 15:20:36 -070018
19 unsigned long flags;
20
21 save_flags(flags); cli();
22 ... critical section ...
23 restore_flags(flags);
24
25so the code does _not_ need to worry about UP vs SMP issues: the spinlocks
26work correctly under both (and spinlocks are actually more efficient on
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050027architectures that allow doing the "save_flags + cli" in one operation).
Linus Torvalds1da177e2005-04-16 15:20:36 -070028
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050029 NOTE! Implications of spin_locks for memory are further described in:
30
31 Documentation/memory-barriers.txt
32 (5) LOCK operations.
33 (6) UNLOCK operations.
Linus Torvalds1da177e2005-04-16 15:20:36 -070034
35The above is usually pretty simple (you usually need and want only one
36spinlock for most things - using more than one spinlock can make things a
37lot more complex and even slower and is usually worth it only for
38sequences that you _know_ need to be split up: avoid it at all cost if you
39aren't sure). HOWEVER, it _does_ mean that if you have some code that does
40
41 cli();
42 .. critical section ..
43 sti();
44
45and another sequence that does
46
47 spin_lock_irqsave(flags);
48 .. critical section ..
49 spin_unlock_irqrestore(flags);
50
51then they are NOT mutually exclusive, and the critical regions can happen
52at the same time on two different CPU's. That's fine per se, but the
53critical regions had better be critical for different things (ie they
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050054can't stomp on each other).
Linus Torvalds1da177e2005-04-16 15:20:36 -070055
56The above is a problem mainly if you end up mixing code - for example the
57routines in ll_rw_block() tend to use cli/sti to protect the atomicity of
58their actions, and if a driver uses spinlocks instead then you should
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050059think about issues like the above.
Linus Torvalds1da177e2005-04-16 15:20:36 -070060
61This is really the only really hard part about spinlocks: once you start
62using spinlocks they tend to expand to areas you might not have noticed
63before, because you have to make sure the spinlocks correctly protect the
64shared data structures _everywhere_ they are used. The spinlocks are most
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050065easily added to places that are completely independent of other code (for
66example, internal driver data structures that nobody else ever touches).
67
68 NOTE! The spin-lock is safe only when you _also_ use the lock itself
69 to do locking across CPU's, which implies that EVERYTHING that
70 touches a shared variable has to agree about the spinlock they want
71 to use.
Linus Torvalds1da177e2005-04-16 15:20:36 -070072
73----
74
75Lesson 2: reader-writer spinlocks.
76
77If your data accesses have a very natural pattern where you usually tend
78to mostly read from the shared variables, the reader-writer locks
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050079(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
Linus Torvalds1da177e2005-04-16 15:20:36 -070080readers to be in the same critical region at once, but if somebody wants
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -050081to change the variables it has to get an exclusive write lock.
82
83 NOTE! reader-writer locks require more atomic memory operations than
84 simple spinlocks. Unless the reader critical section is long, you
85 are better off just using spinlocks.
86
87The routines look the same as above:
Linus Torvalds1da177e2005-04-16 15:20:36 -070088
89 rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
90
Linus Torvalds1da177e2005-04-16 15:20:36 -070091 unsigned long flags;
92
93 read_lock_irqsave(&xxx_lock, flags);
94 .. critical section that only reads the info ...
95 read_unlock_irqrestore(&xxx_lock, flags);
96
97 write_lock_irqsave(&xxx_lock, flags);
98 .. read and write exclusive access to the info ...
99 write_unlock_irqrestore(&xxx_lock, flags);
100
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -0500101The above kind of lock may be useful for complex data structures like
102linked lists, especially searching for entries without changing the list
103itself. The read lock allows many concurrent readers. Anything that
104_changes_ the list will have to get the write lock.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700105
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -0500106 NOTE! RCU is better for list traversal, but requires careful
107 attention to design detail (see Documentation/RCU/listRCU.txt).
108
109Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110time need to do any changes (even if you don't do it every time), you have
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -0500111to get the write-lock at the very beginning.
112
113 NOTE! We are working hard to remove reader-writer spinlocks in most
114 cases, so please don't add a new one without consensus. (Instead, see
115 Documentation/RCU/rcu.txt for complete information.)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116
117----
118
119Lesson 3: spinlocks revisited.
120
121The single spin-lock primitives above are by no means the only ones. They
122are the most safe ones, and the ones that work under all circumstances,
123but partly _because_ they are safe they are also fairly slow. They are
124much faster than a generic global cli/sti pair, but slower than they'd
125need to be, because they do have to disable interrupts (which is just a
126single instruction on a x86, but it's an expensive one - and on other
127architectures it can be worse).
128
129If you have a case where you have to protect a data structure across
130several CPU's and you want to use spinlocks you can potentially use
131cheaper versions of the spinlocks. IFF you know that the spinlocks are
132never used in interrupt handlers, you can use the non-irq versions:
133
134 spin_lock(&lock);
135 ...
136 spin_unlock(&lock);
137
138(and the equivalent read-write versions too, of course). The spinlock will
139guarantee the same kind of exclusive access, and it will be much faster.
140This is useful if you know that the data in question is only ever
141manipulated from a "process context", ie no interrupts involved.
142
143The reasons you mustn't use these versions if you have interrupts that
144play with the spinlock is that you can get deadlocks:
145
146 spin_lock(&lock);
147 ...
148 <- interrupt comes in:
149 spin_lock(&lock);
150
151where an interrupt tries to lock an already locked variable. This is ok if
152the other interrupt happens on another CPU, but it is _not_ ok if the
153interrupt happens on the same CPU that already holds the lock, because the
154lock will obviously never be released (because the interrupt is waiting
155for the lock, and the lock-holder is interrupted by the interrupt and will
156not continue until the interrupt has been processed).
157
158(This is also the reason why the irq-versions of the spinlocks only need
159to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
160on other CPU's, because an interrupt on another CPU doesn't interrupt the
161CPU that holds the lock, so the lock-holder can continue and eventually
162releases the lock).
163
164Note that you can be clever with read-write locks and interrupts. For
165example, if you know that the interrupt only ever gets a read-lock, then
166you can use a non-irq version of read locks everywhere - because they
167don't block on each other (and thus there is no dead-lock wrt interrupts.
168But when you do the write-lock, you have to use the irq-safe version.
169
170For an example of being clever with rw-locks, see the "waitqueue_lock"
171handling in kernel/sched.c - nothing ever _changes_ a wait-queue from
172within an interrupt, they only read the queue in order to know whom to
173wake up. So read-locks are safe (which is good: they are very common
174indeed), while write-locks need to protect themselves against interrupts.
175
176 Linus
177
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -0500178----
Linus Torvalds1da177e2005-04-16 15:20:36 -0700179
William Allen Simpsonfb0bbb92009-12-13 15:12:46 -0500180Reference information:
181
182For dynamic initialization, use spin_lock_init() or rwlock_init() as
183appropriate:
184
185 spinlock_t xxx_lock;
186 rwlock_t xxx_rw_lock;
187
188 static int __init xxx_init(void)
189 {
190 spin_lock_init(&xxx_lock);
191 rwlock_init(&xxx_rw_lock);
192 ...
193 }
194
195 module_init(xxx_init);
196
197For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
198__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
199
200SPIN_LOCK_UNLOCKED and RW_LOCK_UNLOCKED are deprecated. These interfere
201with lockdep state tracking.
202
203Most of the time, you can simply turn:
204 static spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
205into:
206 static DEFINE_SPINLOCK(xxx_lock);
207
208Static structure member variables go from:
209
210 struct foo bar {
211 .lock = SPIN_LOCK_UNLOCKED;
212 };
213
214to:
215
216 struct foo bar {
217 .lock = __SPIN_LOCK_UNLOCKED(bar.lock);
218 };
219
220Declaration of static rw_locks undergo a similar transformation.