Root/Documentation/spinlocks.txt

1Lesson 1: Spin locks
2
3The most basic primitive for locking is spinlock.
4
5static DEFINE_SPINLOCK(xxx_lock);
6
7    unsigned long flags;
8
9    spin_lock_irqsave(&xxx_lock, flags);
10    ... critical section here ..
11    spin_unlock_irqrestore(&xxx_lock, flags);
12
13The above is always safe. It will disable interrupts _locally_, but the
14spinlock 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
16lock. This works well even under UP. The above sequence under UP
17essentially is just the same as doing
18
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
27architectures that allow doing the "save_flags + cli" in one operation).
28
29   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.
34
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
54can't stomp on each other).
55
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
59think about issues like the above.
60
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
65easily 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.
72
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
79(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
80readers to be in the same critical region at once, but if somebody wants
81to 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:
88
89   rwlock_t xxx_lock = RW_LOCK_UNLOCKED;
90
91    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
101The 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.
105
106   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_
110time need to do any changes (even if you don't do it every time), you have
111to 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.)
116
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
178----
179
180Reference 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.
221

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