Root/Documentation/atomic_ops.txt

1        Semantics and Behavior of Atomic and
2                 Bitmask Operations
3
4              David S. Miller
5
6    This document is intended to serve as a guide to Linux port
7maintainers on how to implement atomic counter, bitops, and spinlock
8interfaces properly.
9
10    The atomic_t type should be defined as a signed integer.
11Also, it should be made opaque such that any kind of cast to a normal
12C integer type will fail. Something like the following should
13suffice:
14
15    typedef struct { int counter; } atomic_t;
16
17Historically, counter has been declared volatile. This is now discouraged.
18See Documentation/volatile-considered-harmful.txt for the complete rationale.
19
20local_t is very similar to atomic_t. If the counter is per CPU and only
21updated by one CPU, local_t is probably more appropriate. Please see
22Documentation/local_ops.txt for the semantics of local_t.
23
24The first operations to implement for atomic_t's are the initializers and
25plain reads.
26
27    #define ATOMIC_INIT(i) { (i) }
28    #define atomic_set(v, i) ((v)->counter = (i))
29
30The first macro is used in definitions, such as:
31
32static atomic_t my_counter = ATOMIC_INIT(1);
33
34The initializer is atomic in that the return values of the atomic operations
35are guaranteed to be correct reflecting the initialized value if the
36initializer is used before runtime. If the initializer is used at runtime, a
37proper implicit or explicit read memory barrier is needed before reading the
38value with atomic_read from another thread.
39
40The second interface can be used at runtime, as in:
41
42    struct foo { atomic_t counter; };
43    ...
44
45    struct foo *k;
46
47    k = kmalloc(sizeof(*k), GFP_KERNEL);
48    if (!k)
49        return -ENOMEM;
50    atomic_set(&k->counter, 0);
51
52The setting is atomic in that the return values of the atomic operations by
53all threads are guaranteed to be correct reflecting either the value that has
54been set with this operation or set with another operation. A proper implicit
55or explicit memory barrier is needed before the value set with the operation
56is guaranteed to be readable with atomic_read from another thread.
57
58Next, we have:
59
60    #define atomic_read(v) ((v)->counter)
61
62which simply reads the counter value currently visible to the calling thread.
63The read is atomic in that the return value is guaranteed to be one of the
64values initialized or modified with the interface operations if a proper
65implicit or explicit memory barrier is used after possible runtime
66initialization by any other thread and the value is modified only with the
67interface operations. atomic_read does not guarantee that the runtime
68initialization by any other thread is visible yet, so the user of the
69interface must take care of that with a proper implicit or explicit memory
70barrier.
71
72*** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***
73
74Some architectures may choose to use the volatile keyword, barriers, or inline
75assembly to guarantee some degree of immediacy for atomic_read() and
76atomic_set(). This is not uniformly guaranteed, and may change in the future,
77so all users of atomic_t should treat atomic_read() and atomic_set() as simple
78C statements that may be reordered or optimized away entirely by the compiler
79or processor, and explicitly invoke the appropriate compiler and/or memory
80barrier for each use case. Failure to do so will result in code that may
81suddenly break when used with different architectures or compiler
82optimizations, or even changes in unrelated code which changes how the
83compiler optimizes the section accessing atomic_t variables.
84
85*** YOU HAVE BEEN WARNED! ***
86
87Properly aligned pointers, longs, ints, and chars (and unsigned
88equivalents) may be atomically loaded from and stored to in the same
89sense as described for atomic_read() and atomic_set(). The ACCESS_ONCE()
90macro should be used to prevent the compiler from using optimizations
91that might otherwise optimize accesses out of existence on the one hand,
92or that might create unsolicited accesses on the other.
93
94For example consider the following code:
95
96    while (a > 0)
97        do_something();
98
99If the compiler can prove that do_something() does not store to the
100variable a, then the compiler is within its rights transforming this to
101the following:
102
103    tmp = a;
104    if (a > 0)
105        for (;;)
106            do_something();
107
108If you don't want the compiler to do this (and you probably don't), then
109you should use something like the following:
110
111    while (ACCESS_ONCE(a) < 0)
112        do_something();
113
114Alternatively, you could place a barrier() call in the loop.
115
116For another example, consider the following code:
117
118    tmp_a = a;
119    do_something_with(tmp_a);
120    do_something_else_with(tmp_a);
121
122If the compiler can prove that do_something_with() does not store to the
123variable a, then the compiler is within its rights to manufacture an
124additional load as follows:
125
126    tmp_a = a;
127    do_something_with(tmp_a);
128    tmp_a = a;
129    do_something_else_with(tmp_a);
130
131This could fatally confuse your code if it expected the same value
132to be passed to do_something_with() and do_something_else_with().
133
134The compiler would be likely to manufacture this additional load if
135do_something_with() was an inline function that made very heavy use
136of registers: reloading from variable a could save a flush to the
137stack and later reload. To prevent the compiler from attacking your
138code in this manner, write the following:
139
140    tmp_a = ACCESS_ONCE(a);
141    do_something_with(tmp_a);
142    do_something_else_with(tmp_a);
143
144For a final example, consider the following code, assuming that the
145variable a is set at boot time before the second CPU is brought online
146and never changed later, so that memory barriers are not needed:
147
148    if (a)
149        b = 9;
150    else
151        b = 42;
152
153The compiler is within its rights to manufacture an additional store
154by transforming the above code into the following:
155
156    b = 42;
157    if (a)
158        b = 9;
159
160This could come as a fatal surprise to other code running concurrently
161that expected b to never have the value 42 if a was zero. To prevent
162the compiler from doing this, write something like:
163
164    if (a)
165        ACCESS_ONCE(b) = 9;
166    else
167        ACCESS_ONCE(b) = 42;
168
169Don't even -think- about doing this without proper use of memory barriers,
170locks, or atomic operations if variable a can change at runtime!
171
172*** WARNING: ACCESS_ONCE() DOES NOT IMPLY A BARRIER! ***
173
174Now, we move onto the atomic operation interfaces typically implemented with
175the help of assembly code.
176
177    void atomic_add(int i, atomic_t *v);
178    void atomic_sub(int i, atomic_t *v);
179    void atomic_inc(atomic_t *v);
180    void atomic_dec(atomic_t *v);
181
182These four routines add and subtract integral values to/from the given
183atomic_t value. The first two routines pass explicit integers by
184which to make the adjustment, whereas the latter two use an implicit
185adjustment value of "1".
186
187One very important aspect of these two routines is that they DO NOT
188require any explicit memory barriers. They need only perform the
189atomic_t counter update in an SMP safe manner.
190
191Next, we have:
192
193    int atomic_inc_return(atomic_t *v);
194    int atomic_dec_return(atomic_t *v);
195
196These routines add 1 and subtract 1, respectively, from the given
197atomic_t and return the new counter value after the operation is
198performed.
199
200Unlike the above routines, it is required that explicit memory
201barriers are performed before and after the operation. It must be
202done such that all memory operations before and after the atomic
203operation calls are strongly ordered with respect to the atomic
204operation itself.
205
206For example, it should behave as if a smp_mb() call existed both
207before and after the atomic operation.
208
209If the atomic instructions used in an implementation provide explicit
210memory barrier semantics which satisfy the above requirements, that is
211fine as well.
212
213Let's move on:
214
215    int atomic_add_return(int i, atomic_t *v);
216    int atomic_sub_return(int i, atomic_t *v);
217
218These behave just like atomic_{inc,dec}_return() except that an
219explicit counter adjustment is given instead of the implicit "1".
220This means that like atomic_{inc,dec}_return(), the memory barrier
221semantics are required.
222
223Next:
224
225    int atomic_inc_and_test(atomic_t *v);
226    int atomic_dec_and_test(atomic_t *v);
227
228These two routines increment and decrement by 1, respectively, the
229given atomic counter. They return a boolean indicating whether the
230resulting counter value was zero or not.
231
232It requires explicit memory barrier semantics around the operation as
233above.
234
235    int atomic_sub_and_test(int i, atomic_t *v);
236
237This is identical to atomic_dec_and_test() except that an explicit
238decrement is given instead of the implicit "1". It requires explicit
239memory barrier semantics around the operation.
240
241    int atomic_add_negative(int i, atomic_t *v);
242
243The given increment is added to the given atomic counter value. A
244boolean is return which indicates whether the resulting counter value
245is negative. It requires explicit memory barrier semantics around the
246operation.
247
248Then:
249
250    int atomic_xchg(atomic_t *v, int new);
251
252This performs an atomic exchange operation on the atomic variable v, setting
253the given new value. It returns the old value that the atomic variable v had
254just before the operation.
255
256    int atomic_cmpxchg(atomic_t *v, int old, int new);
257
258This performs an atomic compare exchange operation on the atomic value v,
259with the given old and new values. Like all atomic_xxx operations,
260atomic_cmpxchg will only satisfy its atomicity semantics as long as all
261other accesses of *v are performed through atomic_xxx operations.
262
263atomic_cmpxchg requires explicit memory barriers around the operation.
264
265The semantics for atomic_cmpxchg are the same as those defined for 'cas'
266below.
267
268Finally:
269
270    int atomic_add_unless(atomic_t *v, int a, int u);
271
272If the atomic value v is not equal to u, this function adds a to v, and
273returns non zero. If v is equal to u then it returns zero. This is done as
274an atomic operation.
275
276atomic_add_unless requires explicit memory barriers around the operation
277unless it fails (returns 0).
278
279atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
280
281
282If a caller requires memory barrier semantics around an atomic_t
283operation which does not return a value, a set of interfaces are
284defined which accomplish this:
285
286    void smp_mb__before_atomic_dec(void);
287    void smp_mb__after_atomic_dec(void);
288    void smp_mb__before_atomic_inc(void);
289    void smp_mb__after_atomic_inc(void);
290
291For example, smp_mb__before_atomic_dec() can be used like so:
292
293    obj->dead = 1;
294    smp_mb__before_atomic_dec();
295    atomic_dec(&obj->ref_count);
296
297It makes sure that all memory operations preceding the atomic_dec()
298call are strongly ordered with respect to the atomic counter
299operation. In the above example, it guarantees that the assignment of
300"1" to obj->dead will be globally visible to other cpus before the
301atomic counter decrement.
302
303Without the explicit smp_mb__before_atomic_dec() call, the
304implementation could legally allow the atomic counter update visible
305to other cpus before the "obj->dead = 1;" assignment.
306
307The other three interfaces listed are used to provide explicit
308ordering with respect to memory operations after an atomic_dec() call
309(smp_mb__after_atomic_dec()) and around atomic_inc() calls
310(smp_mb__{before,after}_atomic_inc()).
311
312A missing memory barrier in the cases where they are required by the
313atomic_t implementation above can have disastrous results. Here is
314an example, which follows a pattern occurring frequently in the Linux
315kernel. It is the use of atomic counters to implement reference
316counting, and it works such that once the counter falls to zero it can
317be guaranteed that no other entity can be accessing the object:
318
319static void obj_list_add(struct obj *obj, struct list_head *head)
320{
321    obj->active = 1;
322    list_add(&obj->list, head);
323}
324
325static void obj_list_del(struct obj *obj)
326{
327    list_del(&obj->list);
328    obj->active = 0;
329}
330
331static void obj_destroy(struct obj *obj)
332{
333    BUG_ON(obj->active);
334    kfree(obj);
335}
336
337struct obj *obj_list_peek(struct list_head *head)
338{
339    if (!list_empty(head)) {
340        struct obj *obj;
341
342        obj = list_entry(head->next, struct obj, list);
343        atomic_inc(&obj->refcnt);
344        return obj;
345    }
346    return NULL;
347}
348
349void obj_poke(void)
350{
351    struct obj *obj;
352
353    spin_lock(&global_list_lock);
354    obj = obj_list_peek(&global_list);
355    spin_unlock(&global_list_lock);
356
357    if (obj) {
358        obj->ops->poke(obj);
359        if (atomic_dec_and_test(&obj->refcnt))
360            obj_destroy(obj);
361    }
362}
363
364void obj_timeout(struct obj *obj)
365{
366    spin_lock(&global_list_lock);
367    obj_list_del(obj);
368    spin_unlock(&global_list_lock);
369
370    if (atomic_dec_and_test(&obj->refcnt))
371        obj_destroy(obj);
372}
373
374(This is a simplification of the ARP queue management in the
375 generic neighbour discover code of the networking. Olaf Kirch
376 found a bug wrt. memory barriers in kfree_skb() that exposed
377 the atomic_t memory barrier requirements quite clearly.)
378
379Given the above scheme, it must be the case that the obj->active
380update done by the obj list deletion be visible to other processors
381before the atomic counter decrement is performed.
382
383Otherwise, the counter could fall to zero, yet obj->active would still
384be set, thus triggering the assertion in obj_destroy(). The error
385sequence looks like this:
386
387    cpu 0 cpu 1
388    obj_poke() obj_timeout()
389    obj = obj_list_peek();
390    ... gains ref to obj, refcnt=2
391                    obj_list_del(obj);
392                    obj->active = 0 ...
393                    ... visibility delayed ...
394                    atomic_dec_and_test()
395                    ... refcnt drops to 1 ...
396    atomic_dec_and_test()
397    ... refcount drops to 0 ...
398    obj_destroy()
399    BUG() triggers since obj->active
400    still seen as one
401                    obj->active update visibility occurs
402
403With the memory barrier semantics required of the atomic_t operations
404which return values, the above sequence of memory visibility can never
405happen. Specifically, in the above case the atomic_dec_and_test()
406counter decrement would not become globally visible until the
407obj->active update does.
408
409As a historical note, 32-bit Sparc used to only allow usage of
41024-bits of its atomic_t type. This was because it used 8 bits
411as a spinlock for SMP safety. Sparc32 lacked a "compare and swap"
412type instruction. However, 32-bit Sparc has since been moved over
413to a "hash table of spinlocks" scheme, that allows the full 32-bit
414counter to be realized. Essentially, an array of spinlocks are
415indexed into based upon the address of the atomic_t being operated
416on, and that lock protects the atomic operation. Parisc uses the
417same scheme.
418
419Another note is that the atomic_t operations returning values are
420extremely slow on an old 386.
421
422We will now cover the atomic bitmask operations. You will find that
423their SMP and memory barrier semantics are similar in shape and scope
424to the atomic_t ops above.
425
426Native atomic bit operations are defined to operate on objects aligned
427to the size of an "unsigned long" C data type, and are least of that
428size. The endianness of the bits within each "unsigned long" are the
429native endianness of the cpu.
430
431    void set_bit(unsigned long nr, volatile unsigned long *addr);
432    void clear_bit(unsigned long nr, volatile unsigned long *addr);
433    void change_bit(unsigned long nr, volatile unsigned long *addr);
434
435These routines set, clear, and change, respectively, the bit number
436indicated by "nr" on the bit mask pointed to by "ADDR".
437
438They must execute atomically, yet there are no implicit memory barrier
439semantics required of these interfaces.
440
441    int test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
442    int test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
443    int test_and_change_bit(unsigned long nr, volatile unsigned long *addr);
444
445Like the above, except that these routines return a boolean which
446indicates whether the changed bit was set _BEFORE_ the atomic bit
447operation.
448
449WARNING! It is incredibly important that the value be a boolean,
450ie. "0" or "1". Do not try to be fancy and save a few instructions by
451declaring the above to return "long" and just returning something like
452"old_val & mask" because that will not work.
453
454For one thing, this return value gets truncated to int in many code
455paths using these interfaces, so on 64-bit if the bit is set in the
456upper 32-bits then testers will never see that.
457
458One great example of where this problem crops up are the thread_info
459flag operations. Routines such as test_and_set_ti_thread_flag() chop
460the return value into an int. There are other places where things
461like this occur as well.
462
463These routines, like the atomic_t counter operations returning values,
464require explicit memory barrier semantics around their execution. All
465memory operations before the atomic bit operation call must be made
466visible globally before the atomic bit operation is made visible.
467Likewise, the atomic bit operation must be visible globally before any
468subsequent memory operation is made visible. For example:
469
470    obj->dead = 1;
471    if (test_and_set_bit(0, &obj->flags))
472        /* ... */;
473    obj->killed = 1;
474
475The implementation of test_and_set_bit() must guarantee that
476"obj->dead = 1;" is visible to cpus before the atomic memory operation
477done by test_and_set_bit() becomes visible. Likewise, the atomic
478memory operation done by test_and_set_bit() must become visible before
479"obj->killed = 1;" is visible.
480
481Finally there is the basic operation:
482
483    int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);
484
485Which returns a boolean indicating if bit "nr" is set in the bitmask
486pointed to by "addr".
487
488If explicit memory barriers are required around clear_bit() (which
489does not return a value, and thus does not need to provide memory
490barrier semantics), two interfaces are provided:
491
492    void smp_mb__before_clear_bit(void);
493    void smp_mb__after_clear_bit(void);
494
495They are used as follows, and are akin to their atomic_t operation
496brothers:
497
498    /* All memory operations before this call will
499     * be globally visible before the clear_bit().
500     */
501    smp_mb__before_clear_bit();
502    clear_bit( ... );
503
504    /* The clear_bit() will be visible before all
505     * subsequent memory operations.
506     */
507     smp_mb__after_clear_bit();
508
509There are two special bitops with lock barrier semantics (acquire/release,
510same as spinlocks). These operate in the same way as their non-_lock/unlock
511postfixed variants, except that they are to provide acquire/release semantics,
512respectively. This means they can be used for bit_spin_trylock and
513bit_spin_unlock type operations without specifying any more barriers.
514
515    int test_and_set_bit_lock(unsigned long nr, unsigned long *addr);
516    void clear_bit_unlock(unsigned long nr, unsigned long *addr);
517    void __clear_bit_unlock(unsigned long nr, unsigned long *addr);
518
519The __clear_bit_unlock version is non-atomic, however it still implements
520unlock barrier semantics. This can be useful if the lock itself is protecting
521the other bits in the word.
522
523Finally, there are non-atomic versions of the bitmask operations
524provided. They are used in contexts where some other higher-level SMP
525locking scheme is being used to protect the bitmask, and thus less
526expensive non-atomic operations may be used in the implementation.
527They have names similar to the above bitmask operation interfaces,
528except that two underscores are prefixed to the interface name.
529
530    void __set_bit(unsigned long nr, volatile unsigned long *addr);
531    void __clear_bit(unsigned long nr, volatile unsigned long *addr);
532    void __change_bit(unsigned long nr, volatile unsigned long *addr);
533    int __test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
534    int __test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
535    int __test_and_change_bit(unsigned long nr, volatile unsigned long *addr);
536
537These non-atomic variants also do not require any special memory
538barrier semantics.
539
540The routines xchg() and cmpxchg() need the same exact memory barriers
541as the atomic and bit operations returning values.
542
543Spinlocks and rwlocks have memory barrier expectations as well.
544The rule to follow is simple:
545
5461) When acquiring a lock, the implementation must make it globally
547   visible before any subsequent memory operation.
548
5492) When releasing a lock, the implementation must make it such that
550   all previous memory operations are globally visible before the
551   lock release.
552
553Which finally brings us to _atomic_dec_and_lock(). There is an
554architecture-neutral version implemented in lib/dec_and_lock.c,
555but most platforms will wish to optimize this in assembler.
556
557    int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
558
559Atomically decrement the given counter, and if will drop to zero
560atomically acquire the given spinlock and perform the decrement
561of the counter to zero. If it does not drop to zero, do nothing
562with the spinlock.
563
564It is actually pretty simple to get the memory barrier correct.
565Simply satisfy the spinlock grab requirements, which is make
566sure the spinlock operation is globally visible before any
567subsequent memory operation.
568
569We can demonstrate this operation more clearly if we define
570an abstract atomic operation:
571
572    long cas(long *mem, long old, long new);
573
574"cas" stands for "compare and swap". It atomically:
575
5761) Compares "old" with the value currently at "mem".
5772) If they are equal, "new" is written to "mem".
5783) Regardless, the current value at "mem" is returned.
579
580As an example usage, here is what an atomic counter update
581might look like:
582
583void example_atomic_inc(long *counter)
584{
585    long old, new, ret;
586
587    while (1) {
588        old = *counter;
589        new = old + 1;
590
591        ret = cas(counter, old, new);
592        if (ret == old)
593            break;
594    }
595}
596
597Let's use cas() in order to build a pseudo-C atomic_dec_and_lock():
598
599int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
600{
601    long old, new, ret;
602    int went_to_zero;
603
604    went_to_zero = 0;
605    while (1) {
606        old = atomic_read(atomic);
607        new = old - 1;
608        if (new == 0) {
609            went_to_zero = 1;
610            spin_lock(lock);
611        }
612        ret = cas(atomic, old, new);
613        if (ret == old)
614            break;
615        if (went_to_zero) {
616            spin_unlock(lock);
617            went_to_zero = 0;
618        }
619    }
620
621    return went_to_zero;
622}
623
624Now, as far as memory barriers go, as long as spin_lock()
625strictly orders all subsequent memory operations (including
626the cas()) with respect to itself, things will be fine.
627
628Said another way, _atomic_dec_and_lock() must guarantee that
629a counter dropping to zero is never made visible before the
630spinlock being acquired.
631
632Note that this also means that for the case where the counter
633is not dropping to zero, there are no memory ordering
634requirements.
635

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