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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 { volatile int counter; } atomic_t;
16
17 The first operations to implement for atomic_t's are the
18initializers and plain reads.
19
20 #define ATOMIC_INIT(i) { (i) }
21 #define atomic_set(v, i) ((v)->counter = (i))
22
23The first macro is used in definitions, such as:
24
25static atomic_t my_counter = ATOMIC_INIT(1);
26
27The second interface can be used at runtime, as in:
28
29 struct foo { atomic_t counter; };
30 ...
31
32 struct foo *k;
33
34 k = kmalloc(sizeof(*k), GFP_KERNEL);
35 if (!k)
36 return -ENOMEM;
37 atomic_set(&k->counter, 0);
38
39Next, we have:
40
41 #define atomic_read(v) ((v)->counter)
42
43which simply reads the current value of the counter.
44
45Now, we move onto the actual atomic operation interfaces.
46
47 void atomic_add(int i, atomic_t *v);
48 void atomic_sub(int i, atomic_t *v);
49 void atomic_inc(atomic_t *v);
50 void atomic_dec(atomic_t *v);
51
52These four routines add and subtract integral values to/from the given
53atomic_t value. The first two routines pass explicit integers by
54which to make the adjustment, whereas the latter two use an implicit
55adjustment value of "1".
56
57One very important aspect of these two routines is that they DO NOT
58require any explicit memory barriers. They need only perform the
59atomic_t counter update in an SMP safe manner.
60
61Next, we have:
62
63 int atomic_inc_return(atomic_t *v);
64 int atomic_dec_return(atomic_t *v);
65
66These routines add 1 and subtract 1, respectively, from the given
67atomic_t and return the new counter value after the operation is
68performed.
69
70Unlike the above routines, it is required that explicit memory
71barriers are performed before and after the operation. It must be
72done such that all memory operations before and after the atomic
73operation calls are strongly ordered with respect to the atomic
74operation itself.
75
76For example, it should behave as if a smp_mb() call existed both
77before and after the atomic operation.
78
79If the atomic instructions used in an implementation provide explicit
80memory barrier semantics which satisfy the above requirements, that is
81fine as well.
82
83Let's move on:
84
85 int atomic_add_return(int i, atomic_t *v);
86 int atomic_sub_return(int i, atomic_t *v);
87
88These behave just like atomic_{inc,dec}_return() except that an
89explicit counter adjustment is given instead of the implicit "1".
90This means that like atomic_{inc,dec}_return(), the memory barrier
91semantics are required.
92
93Next:
94
95 int atomic_inc_and_test(atomic_t *v);
96 int atomic_dec_and_test(atomic_t *v);
97
98These two routines increment and decrement by 1, respectively, the
99given atomic counter. They return a boolean indicating whether the
100resulting counter value was zero or not.
101
102It requires explicit memory barrier semantics around the operation as
103above.
104
105 int atomic_sub_and_test(int i, atomic_t *v);
106
107This is identical to atomic_dec_and_test() except that an explicit
108decrement is given instead of the implicit "1". It requires explicit
109memory barrier semantics around the operation.
110
111 int atomic_add_negative(int i, atomic_t *v);
112
113The given increment is added to the given atomic counter value. A
114boolean is return which indicates whether the resulting counter value
115is negative. It requires explicit memory barrier semantics around the
116operation.
117
118If a caller requires memory barrier semantics around an atomic_t
119operation which does not return a value, a set of interfaces are
120defined which accomplish this:
121
122 void smp_mb__before_atomic_dec(void);
123 void smp_mb__after_atomic_dec(void);
124 void smp_mb__before_atomic_inc(void);
125 void smp_mb__after_atomic_dec(void);
126
127For example, smp_mb__before_atomic_dec() can be used like so:
128
129 obj->dead = 1;
130 smp_mb__before_atomic_dec();
131 atomic_dec(&obj->ref_count);
132
133It makes sure that all memory operations preceeding the atomic_dec()
134call are strongly ordered with respect to the atomic counter
135operation. In the above example, it guarentees that the assignment of
136"1" to obj->dead will be globally visible to other cpus before the
137atomic counter decrement.
138
139Without the explicitl smp_mb__before_atomic_dec() call, the
140implementation could legally allow the atomic counter update visible
141to other cpus before the "obj->dead = 1;" assignment.
142
143The other three interfaces listed are used to provide explicit
144ordering with respect to memory operations after an atomic_dec() call
145(smp_mb__after_atomic_dec()) and around atomic_inc() calls
146(smp_mb__{before,after}_atomic_inc()).
147
148A missing memory barrier in the cases where they are required by the
149atomic_t implementation above can have disasterous results. Here is
150an example, which follows a pattern occuring frequently in the Linux
151kernel. It is the use of atomic counters to implement reference
152counting, and it works such that once the counter falls to zero it can
153be guarenteed that no other entity can be accessing the object:
154
155static void obj_list_add(struct obj *obj)
156{
157 obj->active = 1;
158 list_add(&obj->list);
159}
160
161static void obj_list_del(struct obj *obj)
162{
163 list_del(&obj->list);
164 obj->active = 0;
165}
166
167static void obj_destroy(struct obj *obj)
168{
169 BUG_ON(obj->active);
170 kfree(obj);
171}
172
173struct obj *obj_list_peek(struct list_head *head)
174{
175 if (!list_empty(head)) {
176 struct obj *obj;
177
178 obj = list_entry(head->next, struct obj, list);
179 atomic_inc(&obj->refcnt);
180 return obj;
181 }
182 return NULL;
183}
184
185void obj_poke(void)
186{
187 struct obj *obj;
188
189 spin_lock(&global_list_lock);
190 obj = obj_list_peek(&global_list);
191 spin_unlock(&global_list_lock);
192
193 if (obj) {
194 obj->ops->poke(obj);
195 if (atomic_dec_and_test(&obj->refcnt))
196 obj_destroy(obj);
197 }
198}
199
200void obj_timeout(struct obj *obj)
201{
202 spin_lock(&global_list_lock);
203 obj_list_del(obj);
204 spin_unlock(&global_list_lock);
205
206 if (atomic_dec_and_test(&obj->refcnt))
207 obj_destroy(obj);
208}
209
210(This is a simplification of the ARP queue management in the
211 generic neighbour discover code of the networking. Olaf Kirch
212 found a bug wrt. memory barriers in kfree_skb() that exposed
213 the atomic_t memory barrier requirements quite clearly.)
214
215Given the above scheme, it must be the case that the obj->active
216update done by the obj list deletion be visible to other processors
217before the atomic counter decrement is performed.
218
219Otherwise, the counter could fall to zero, yet obj->active would still
220be set, thus triggering the assertion in obj_destroy(). The error
221sequence looks like this:
222
223 cpu 0 cpu 1
224 obj_poke() obj_timeout()
225 obj = obj_list_peek();
226 ... gains ref to obj, refcnt=2
227 obj_list_del(obj);
228 obj->active = 0 ...
229 ... visibility delayed ...
230 atomic_dec_and_test()
231 ... refcnt drops to 1 ...
232 atomic_dec_and_test()
233 ... refcount drops to 0 ...
234 obj_destroy()
235 BUG() triggers since obj->active
236 still seen as one
237 obj->active update visibility occurs
238
239With the memory barrier semantics required of the atomic_t operations
240which return values, the above sequence of memory visibility can never
241happen. Specifically, in the above case the atomic_dec_and_test()
242counter decrement would not become globally visible until the
243obj->active update does.
244
245As a historical note, 32-bit Sparc used to only allow usage of
24624-bits of it's atomic_t type. This was because it used 8 bits
247as a spinlock for SMP safety. Sparc32 lacked a "compare and swap"
248type instruction. However, 32-bit Sparc has since been moved over
249to a "hash table of spinlocks" scheme, that allows the full 32-bit
250counter to be realized. Essentially, an array of spinlocks are
251indexed into based upon the address of the atomic_t being operated
252on, and that lock protects the atomic operation. Parisc uses the
253same scheme.
254
255Another note is that the atomic_t operations returning values are
256extremely slow on an old 386.
257
258We will now cover the atomic bitmask operations. You will find that
259their SMP and memory barrier semantics are similar in shape and scope
260to the atomic_t ops above.
261
262Native atomic bit operations are defined to operate on objects aligned
263to the size of an "unsigned long" C data type, and are least of that
264size. The endianness of the bits within each "unsigned long" are the
265native endianness of the cpu.
266
267 void set_bit(unsigned long nr, volatils unsigned long *addr);
268 void clear_bit(unsigned long nr, volatils unsigned long *addr);
269 void change_bit(unsigned long nr, volatils unsigned long *addr);
270
271These routines set, clear, and change, respectively, the bit number
272indicated by "nr" on the bit mask pointed to by "ADDR".
273
274They must execute atomically, yet there are no implicit memory barrier
275semantics required of these interfaces.
276
277 int test_and_set_bit(unsigned long nr, volatils unsigned long *addr);
278 int test_and_clear_bit(unsigned long nr, volatils unsigned long *addr);
279 int test_and_change_bit(unsigned long nr, volatils unsigned long *addr);
280
281Like the above, except that these routines return a boolean which
282indicates whether the changed bit was set _BEFORE_ the atomic bit
283operation.
284
285WARNING! It is incredibly important that the value be a boolean,
286ie. "0" or "1". Do not try to be fancy and save a few instructions by
287declaring the above to return "long" and just returning something like
288"old_val & mask" because that will not work.
289
290For one thing, this return value gets truncated to int in many code
291paths using these interfaces, so on 64-bit if the bit is set in the
292upper 32-bits then testers will never see that.
293
294One great example of where this problem crops up are the thread_info
295flag operations. Routines such as test_and_set_ti_thread_flag() chop
296the return value into an int. There are other places where things
297like this occur as well.
298
299These routines, like the atomic_t counter operations returning values,
300require explicit memory barrier semantics around their execution. All
301memory operations before the atomic bit operation call must be made
302visible globally before the atomic bit operation is made visible.
303Likewise, the atomic bit operation must be visible globally before any
304subsequent memory operation is made visible. For example:
305
306 obj->dead = 1;
307 if (test_and_set_bit(0, &obj->flags))
308 /* ... */;
309 obj->killed = 1;
310
311The implementation of test_and_set_bit() must guarentee that
312"obj->dead = 1;" is visible to cpus before the atomic memory operation
313done by test_and_set_bit() becomes visible. Likewise, the atomic
314memory operation done by test_and_set_bit() must become visible before
315"obj->killed = 1;" is visible.
316
317Finally there is the basic operation:
318
319 int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);
320
321Which returns a boolean indicating if bit "nr" is set in the bitmask
322pointed to by "addr".
323
324If explicit memory barriers are required around clear_bit() (which
325does not return a value, and thus does not need to provide memory
326barrier semantics), two interfaces are provided:
327
328 void smp_mb__before_clear_bit(void);
329 void smp_mb__after_clear_bit(void);
330
331They are used as follows, and are akin to their atomic_t operation
332brothers:
333
334 /* All memory operations before this call will
335 * be globally visible before the clear_bit().
336 */
337 smp_mb__before_clear_bit();
338 clear_bit( ... );
339
340 /* The clear_bit() will be visible before all
341 * subsequent memory operations.
342 */
343 smp_mb__after_clear_bit();
344
345Finally, there are non-atomic versions of the bitmask operations
346provided. They are used in contexts where some other higher-level SMP
347locking scheme is being used to protect the bitmask, and thus less
348expensive non-atomic operations may be used in the implementation.
349They have names similar to the above bitmask operation interfaces,
350except that two underscores are prefixed to the interface name.
351
352 void __set_bit(unsigned long nr, volatile unsigned long *addr);
353 void __clear_bit(unsigned long nr, volatile unsigned long *addr);
354 void __change_bit(unsigned long nr, volatile unsigned long *addr);
355 int __test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
356 int __test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
357 int __test_and_change_bit(unsigned long nr, volatile unsigned long *addr);
358
359These non-atomic variants also do not require any special memory
360barrier semantics.
361
362The routines xchg() and cmpxchg() need the same exact memory barriers
363as the atomic and bit operations returning values.
364
365Spinlocks and rwlocks have memory barrier expectations as well.
366The rule to follow is simple:
367
3681) When acquiring a lock, the implementation must make it globally
369 visible before any subsequent memory operation.
370
3712) When releasing a lock, the implementation must make it such that
372 all previous memory operations are globally visible before the
373 lock release.
374
375Which finally brings us to _atomic_dec_and_lock(). There is an
376architecture-neutral version implemented in lib/dec_and_lock.c,
377but most platforms will wish to optimize this in assembler.
378
379 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
380
381Atomically decrement the given counter, and if will drop to zero
382atomically acquire the given spinlock and perform the decrement
383of the counter to zero. If it does not drop to zero, do nothing
384with the spinlock.
385
386It is actually pretty simple to get the memory barrier correct.
387Simply satisfy the spinlock grab requirements, which is make
388sure the spinlock operation is globally visible before any
389subsequent memory operation.
390
391We can demonstrate this operation more clearly if we define
392an abstract atomic operation:
393
394 long cas(long *mem, long old, long new);
395
396"cas" stands for "compare and swap". It atomically:
397
3981) Compares "old" with the value currently at "mem".
3992) If they are equal, "new" is written to "mem".
4003) Regardless, the current value at "mem" is returned.
401
402As an example usage, here is what an atomic counter update
403might look like:
404
405void example_atomic_inc(long *counter)
406{
407 long old, new, ret;
408
409 while (1) {
410 old = *counter;
411 new = old + 1;
412
413 ret = cas(counter, old, new);
414 if (ret == old)
415 break;
416 }
417}
418
419Let's use cas() in order to build a pseudo-C atomic_dec_and_lock():
420
421int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
422{
423 long old, new, ret;
424 int went_to_zero;
425
426 went_to_zero = 0;
427 while (1) {
428 old = atomic_read(atomic);
429 new = old - 1;
430 if (new == 0) {
431 went_to_zero = 1;
432 spin_lock(lock);
433 }
434 ret = cas(atomic, old, new);
435 if (ret == old)
436 break;
437 if (went_to_zero) {
438 spin_unlock(lock);
439 went_to_zero = 0;
440 }
441 }
442
443 return went_to_zero;
444}
445
446Now, as far as memory barriers go, as long as spin_lock()
447strictly orders all subsequent memory operations (including
448the cas()) with respect to itself, things will be fine.
449
450Said another way, _atomic_dec_and_lock() must guarentee that
451a counter dropping to zero is never made visible before the
452spinlock being acquired.
453
454Note that this also means that for the case where the counter
455is not dropping to zero, there are no memory ordering
456requirements.