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1da177e4 LT |
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 | |
7 | maintainers on how to implement atomic counter, bitops, and spinlock | |
8 | interfaces properly. | |
9 | ||
10 | The atomic_t type should be defined as a signed integer. | |
11 | Also, it should be made opaque such that any kind of cast to a normal | |
12 | C integer type will fail. Something like the following should | |
13 | suffice: | |
14 | ||
15 | typedef struct { volatile int counter; } atomic_t; | |
16 | ||
17 | The first operations to implement for atomic_t's are the | |
18 | initializers and plain reads. | |
19 | ||
20 | #define ATOMIC_INIT(i) { (i) } | |
21 | #define atomic_set(v, i) ((v)->counter = (i)) | |
22 | ||
23 | The first macro is used in definitions, such as: | |
24 | ||
25 | static atomic_t my_counter = ATOMIC_INIT(1); | |
26 | ||
27 | The 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 | ||
39 | Next, we have: | |
40 | ||
41 | #define atomic_read(v) ((v)->counter) | |
42 | ||
43 | which simply reads the current value of the counter. | |
44 | ||
45 | Now, 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 | ||
52 | These four routines add and subtract integral values to/from the given | |
53 | atomic_t value. The first two routines pass explicit integers by | |
54 | which to make the adjustment, whereas the latter two use an implicit | |
55 | adjustment value of "1". | |
56 | ||
57 | One very important aspect of these two routines is that they DO NOT | |
58 | require any explicit memory barriers. They need only perform the | |
59 | atomic_t counter update in an SMP safe manner. | |
60 | ||
61 | Next, we have: | |
62 | ||
63 | int atomic_inc_return(atomic_t *v); | |
64 | int atomic_dec_return(atomic_t *v); | |
65 | ||
66 | These routines add 1 and subtract 1, respectively, from the given | |
67 | atomic_t and return the new counter value after the operation is | |
68 | performed. | |
69 | ||
70 | Unlike the above routines, it is required that explicit memory | |
71 | barriers are performed before and after the operation. It must be | |
72 | done such that all memory operations before and after the atomic | |
73 | operation calls are strongly ordered with respect to the atomic | |
74 | operation itself. | |
75 | ||
76 | For example, it should behave as if a smp_mb() call existed both | |
77 | before and after the atomic operation. | |
78 | ||
79 | If the atomic instructions used in an implementation provide explicit | |
80 | memory barrier semantics which satisfy the above requirements, that is | |
81 | fine as well. | |
82 | ||
83 | Let'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 | ||
88 | These behave just like atomic_{inc,dec}_return() except that an | |
89 | explicit counter adjustment is given instead of the implicit "1". | |
90 | This means that like atomic_{inc,dec}_return(), the memory barrier | |
91 | semantics are required. | |
92 | ||
93 | Next: | |
94 | ||
95 | int atomic_inc_and_test(atomic_t *v); | |
96 | int atomic_dec_and_test(atomic_t *v); | |
97 | ||
98 | These two routines increment and decrement by 1, respectively, the | |
99 | given atomic counter. They return a boolean indicating whether the | |
100 | resulting counter value was zero or not. | |
101 | ||
102 | It requires explicit memory barrier semantics around the operation as | |
103 | above. | |
104 | ||
105 | int atomic_sub_and_test(int i, atomic_t *v); | |
106 | ||
107 | This is identical to atomic_dec_and_test() except that an explicit | |
108 | decrement is given instead of the implicit "1". It requires explicit | |
109 | memory barrier semantics around the operation. | |
110 | ||
111 | int atomic_add_negative(int i, atomic_t *v); | |
112 | ||
113 | The given increment is added to the given atomic counter value. A | |
114 | boolean is return which indicates whether the resulting counter value | |
115 | is negative. It requires explicit memory barrier semantics around the | |
116 | operation. | |
117 | ||
118 | If a caller requires memory barrier semantics around an atomic_t | |
119 | operation which does not return a value, a set of interfaces are | |
120 | defined 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 | ||
127 | For 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 | ||
133 | It makes sure that all memory operations preceeding the atomic_dec() | |
134 | call are strongly ordered with respect to the atomic counter | |
135 | operation. In the above example, it guarentees that the assignment of | |
136 | "1" to obj->dead will be globally visible to other cpus before the | |
137 | atomic counter decrement. | |
138 | ||
139 | Without the explicitl smp_mb__before_atomic_dec() call, the | |
140 | implementation could legally allow the atomic counter update visible | |
141 | to other cpus before the "obj->dead = 1;" assignment. | |
142 | ||
143 | The other three interfaces listed are used to provide explicit | |
144 | ordering 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 | ||
148 | A missing memory barrier in the cases where they are required by the | |
149 | atomic_t implementation above can have disasterous results. Here is | |
150 | an example, which follows a pattern occuring frequently in the Linux | |
151 | kernel. It is the use of atomic counters to implement reference | |
152 | counting, and it works such that once the counter falls to zero it can | |
153 | be guarenteed that no other entity can be accessing the object: | |
154 | ||
155 | static void obj_list_add(struct obj *obj) | |
156 | { | |
157 | obj->active = 1; | |
158 | list_add(&obj->list); | |
159 | } | |
160 | ||
161 | static void obj_list_del(struct obj *obj) | |
162 | { | |
163 | list_del(&obj->list); | |
164 | obj->active = 0; | |
165 | } | |
166 | ||
167 | static void obj_destroy(struct obj *obj) | |
168 | { | |
169 | BUG_ON(obj->active); | |
170 | kfree(obj); | |
171 | } | |
172 | ||
173 | struct 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 | ||
185 | void 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 | ||
200 | void 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 | ||
215 | Given the above scheme, it must be the case that the obj->active | |
216 | update done by the obj list deletion be visible to other processors | |
217 | before the atomic counter decrement is performed. | |
218 | ||
219 | Otherwise, the counter could fall to zero, yet obj->active would still | |
220 | be set, thus triggering the assertion in obj_destroy(). The error | |
221 | sequence 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 | ||
239 | With the memory barrier semantics required of the atomic_t operations | |
240 | which return values, the above sequence of memory visibility can never | |
241 | happen. Specifically, in the above case the atomic_dec_and_test() | |
242 | counter decrement would not become globally visible until the | |
243 | obj->active update does. | |
244 | ||
245 | As a historical note, 32-bit Sparc used to only allow usage of | |
246 | 24-bits of it's atomic_t type. This was because it used 8 bits | |
247 | as a spinlock for SMP safety. Sparc32 lacked a "compare and swap" | |
248 | type instruction. However, 32-bit Sparc has since been moved over | |
249 | to a "hash table of spinlocks" scheme, that allows the full 32-bit | |
250 | counter to be realized. Essentially, an array of spinlocks are | |
251 | indexed into based upon the address of the atomic_t being operated | |
252 | on, and that lock protects the atomic operation. Parisc uses the | |
253 | same scheme. | |
254 | ||
255 | Another note is that the atomic_t operations returning values are | |
256 | extremely slow on an old 386. | |
257 | ||
258 | We will now cover the atomic bitmask operations. You will find that | |
259 | their SMP and memory barrier semantics are similar in shape and scope | |
260 | to the atomic_t ops above. | |
261 | ||
262 | Native atomic bit operations are defined to operate on objects aligned | |
263 | to the size of an "unsigned long" C data type, and are least of that | |
264 | size. The endianness of the bits within each "unsigned long" are the | |
265 | native 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 | ||
271 | These routines set, clear, and change, respectively, the bit number | |
272 | indicated by "nr" on the bit mask pointed to by "ADDR". | |
273 | ||
274 | They must execute atomically, yet there are no implicit memory barrier | |
275 | semantics 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 | ||
281 | Like the above, except that these routines return a boolean which | |
282 | indicates whether the changed bit was set _BEFORE_ the atomic bit | |
283 | operation. | |
284 | ||
285 | WARNING! It is incredibly important that the value be a boolean, | |
286 | ie. "0" or "1". Do not try to be fancy and save a few instructions by | |
287 | declaring the above to return "long" and just returning something like | |
288 | "old_val & mask" because that will not work. | |
289 | ||
290 | For one thing, this return value gets truncated to int in many code | |
291 | paths using these interfaces, so on 64-bit if the bit is set in the | |
292 | upper 32-bits then testers will never see that. | |
293 | ||
294 | One great example of where this problem crops up are the thread_info | |
295 | flag operations. Routines such as test_and_set_ti_thread_flag() chop | |
296 | the return value into an int. There are other places where things | |
297 | like this occur as well. | |
298 | ||
299 | These routines, like the atomic_t counter operations returning values, | |
300 | require explicit memory barrier semantics around their execution. All | |
301 | memory operations before the atomic bit operation call must be made | |
302 | visible globally before the atomic bit operation is made visible. | |
303 | Likewise, the atomic bit operation must be visible globally before any | |
304 | subsequent 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 | ||
311 | The implementation of test_and_set_bit() must guarentee that | |
312 | "obj->dead = 1;" is visible to cpus before the atomic memory operation | |
313 | done by test_and_set_bit() becomes visible. Likewise, the atomic | |
314 | memory operation done by test_and_set_bit() must become visible before | |
315 | "obj->killed = 1;" is visible. | |
316 | ||
317 | Finally there is the basic operation: | |
318 | ||
319 | int test_bit(unsigned long nr, __const__ volatile unsigned long *addr); | |
320 | ||
321 | Which returns a boolean indicating if bit "nr" is set in the bitmask | |
322 | pointed to by "addr". | |
323 | ||
324 | If explicit memory barriers are required around clear_bit() (which | |
325 | does not return a value, and thus does not need to provide memory | |
326 | barrier semantics), two interfaces are provided: | |
327 | ||
328 | void smp_mb__before_clear_bit(void); | |
329 | void smp_mb__after_clear_bit(void); | |
330 | ||
331 | They are used as follows, and are akin to their atomic_t operation | |
332 | brothers: | |
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 | ||
345 | Finally, there are non-atomic versions of the bitmask operations | |
346 | provided. They are used in contexts where some other higher-level SMP | |
347 | locking scheme is being used to protect the bitmask, and thus less | |
348 | expensive non-atomic operations may be used in the implementation. | |
349 | They have names similar to the above bitmask operation interfaces, | |
350 | except 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 | ||
359 | These non-atomic variants also do not require any special memory | |
360 | barrier semantics. | |
361 | ||
362 | The routines xchg() and cmpxchg() need the same exact memory barriers | |
363 | as the atomic and bit operations returning values. | |
364 | ||
365 | Spinlocks and rwlocks have memory barrier expectations as well. | |
366 | The rule to follow is simple: | |
367 | ||
368 | 1) When acquiring a lock, the implementation must make it globally | |
369 | visible before any subsequent memory operation. | |
370 | ||
371 | 2) 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 | ||
375 | Which finally brings us to _atomic_dec_and_lock(). There is an | |
376 | architecture-neutral version implemented in lib/dec_and_lock.c, | |
377 | but most platforms will wish to optimize this in assembler. | |
378 | ||
379 | int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock); | |
380 | ||
381 | Atomically decrement the given counter, and if will drop to zero | |
382 | atomically acquire the given spinlock and perform the decrement | |
383 | of the counter to zero. If it does not drop to zero, do nothing | |
384 | with the spinlock. | |
385 | ||
386 | It is actually pretty simple to get the memory barrier correct. | |
387 | Simply satisfy the spinlock grab requirements, which is make | |
388 | sure the spinlock operation is globally visible before any | |
389 | subsequent memory operation. | |
390 | ||
391 | We can demonstrate this operation more clearly if we define | |
392 | an abstract atomic operation: | |
393 | ||
394 | long cas(long *mem, long old, long new); | |
395 | ||
396 | "cas" stands for "compare and swap". It atomically: | |
397 | ||
398 | 1) Compares "old" with the value currently at "mem". | |
399 | 2) If they are equal, "new" is written to "mem". | |
400 | 3) Regardless, the current value at "mem" is returned. | |
401 | ||
402 | As an example usage, here is what an atomic counter update | |
403 | might look like: | |
404 | ||
405 | void 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 | ||
419 | Let's use cas() in order to build a pseudo-C atomic_dec_and_lock(): | |
420 | ||
421 | int _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 | ||
446 | Now, as far as memory barriers go, as long as spin_lock() | |
447 | strictly orders all subsequent memory operations (including | |
448 | the cas()) with respect to itself, things will be fine. | |
449 | ||
450 | Said another way, _atomic_dec_and_lock() must guarentee that | |
451 | a counter dropping to zero is never made visible before the | |
452 | spinlock being acquired. | |
453 | ||
454 | Note that this also means that for the case where the counter | |
455 | is not dropping to zero, there are no memory ordering | |
456 | requirements. |