view Documentation/atomic_ops.txt @ 452:c7ed6fe5dca0

kexec: dont initialise regions in reserve_memory()

There is no need to initialise efi_memmap_res and boot_param_res in
reserve_memory() for the initial xen domain as it is done in
machine_kexec_setup_resources() using values from the kexec hypercall.

Signed-off-by: Simon Horman <horms@verge.net.au>
author Keir Fraser <keir.fraser@citrix.com>
date Thu Feb 28 10:55:18 2008 +0000 (2008-02-28)
parents 831230e53067
line source
1 Semantics and Behavior of Atomic and
2 Bitmask Operations
4 David S. Miller
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.
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:
15 typedef struct { volatile int counter; } atomic_t;
17 The first operations to implement for atomic_t's are the
18 initializers and plain reads.
20 #define ATOMIC_INIT(i) { (i) }
21 #define atomic_set(v, i) ((v)->counter = (i))
23 The first macro is used in definitions, such as:
25 static atomic_t my_counter = ATOMIC_INIT(1);
27 The second interface can be used at runtime, as in:
29 struct foo { atomic_t counter; };
30 ...
32 struct foo *k;
34 k = kmalloc(sizeof(*k), GFP_KERNEL);
35 if (!k)
36 return -ENOMEM;
37 atomic_set(&k->counter, 0);
39 Next, we have:
41 #define atomic_read(v) ((v)->counter)
43 which simply reads the current value of the counter.
45 Now, we move onto the actual atomic operation interfaces.
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);
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".
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.
61 Next, we have:
63 int atomic_inc_return(atomic_t *v);
64 int atomic_dec_return(atomic_t *v);
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.
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.
76 For example, it should behave as if a smp_mb() call existed both
77 before and after the atomic operation.
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.
83 Let's move on:
85 int atomic_add_return(int i, atomic_t *v);
86 int atomic_sub_return(int i, atomic_t *v);
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.
93 Next:
95 int atomic_inc_and_test(atomic_t *v);
96 int atomic_dec_and_test(atomic_t *v);
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.
102 It requires explicit memory barrier semantics around the operation as
103 above.
105 int atomic_sub_and_test(int i, atomic_t *v);
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.
111 int atomic_add_negative(int i, atomic_t *v);
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.
118 Then:
120 int atomic_cmpxchg(atomic_t *v, int old, int new);
122 This performs an atomic compare exchange operation on the atomic value v,
123 with the given old and new values. Like all atomic_xxx operations,
124 atomic_cmpxchg will only satisfy its atomicity semantics as long as all
125 other accesses of *v are performed through atomic_xxx operations.
127 atomic_cmpxchg requires explicit memory barriers around the operation.
129 The semantics for atomic_cmpxchg are the same as those defined for 'cas'
130 below.
132 Finally:
134 int atomic_add_unless(atomic_t *v, int a, int u);
136 If the atomic value v is not equal to u, this function adds a to v, and
137 returns non zero. If v is equal to u then it returns zero. This is done as
138 an atomic operation.
140 atomic_add_unless requires explicit memory barriers around the operation.
142 atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
145 If a caller requires memory barrier semantics around an atomic_t
146 operation which does not return a value, a set of interfaces are
147 defined which accomplish this:
149 void smp_mb__before_atomic_dec(void);
150 void smp_mb__after_atomic_dec(void);
151 void smp_mb__before_atomic_inc(void);
152 void smp_mb__after_atomic_dec(void);
154 For example, smp_mb__before_atomic_dec() can be used like so:
156 obj->dead = 1;
157 smp_mb__before_atomic_dec();
158 atomic_dec(&obj->ref_count);
160 It makes sure that all memory operations preceding the atomic_dec()
161 call are strongly ordered with respect to the atomic counter
162 operation. In the above example, it guarantees that the assignment of
163 "1" to obj->dead will be globally visible to other cpus before the
164 atomic counter decrement.
166 Without the explicit smp_mb__before_atomic_dec() call, the
167 implementation could legally allow the atomic counter update visible
168 to other cpus before the "obj->dead = 1;" assignment.
170 The other three interfaces listed are used to provide explicit
171 ordering with respect to memory operations after an atomic_dec() call
172 (smp_mb__after_atomic_dec()) and around atomic_inc() calls
173 (smp_mb__{before,after}_atomic_inc()).
175 A missing memory barrier in the cases where they are required by the
176 atomic_t implementation above can have disastrous results. Here is
177 an example, which follows a pattern occurring frequently in the Linux
178 kernel. It is the use of atomic counters to implement reference
179 counting, and it works such that once the counter falls to zero it can
180 be guaranteed that no other entity can be accessing the object:
182 static void obj_list_add(struct obj *obj)
183 {
184 obj->active = 1;
185 list_add(&obj->list);
186 }
188 static void obj_list_del(struct obj *obj)
189 {
190 list_del(&obj->list);
191 obj->active = 0;
192 }
194 static void obj_destroy(struct obj *obj)
195 {
196 BUG_ON(obj->active);
197 kfree(obj);
198 }
200 struct obj *obj_list_peek(struct list_head *head)
201 {
202 if (!list_empty(head)) {
203 struct obj *obj;
205 obj = list_entry(head->next, struct obj, list);
206 atomic_inc(&obj->refcnt);
207 return obj;
208 }
209 return NULL;
210 }
212 void obj_poke(void)
213 {
214 struct obj *obj;
216 spin_lock(&global_list_lock);
217 obj = obj_list_peek(&global_list);
218 spin_unlock(&global_list_lock);
220 if (obj) {
221 obj->ops->poke(obj);
222 if (atomic_dec_and_test(&obj->refcnt))
223 obj_destroy(obj);
224 }
225 }
227 void obj_timeout(struct obj *obj)
228 {
229 spin_lock(&global_list_lock);
230 obj_list_del(obj);
231 spin_unlock(&global_list_lock);
233 if (atomic_dec_and_test(&obj->refcnt))
234 obj_destroy(obj);
235 }
237 (This is a simplification of the ARP queue management in the
238 generic neighbour discover code of the networking. Olaf Kirch
239 found a bug wrt. memory barriers in kfree_skb() that exposed
240 the atomic_t memory barrier requirements quite clearly.)
242 Given the above scheme, it must be the case that the obj->active
243 update done by the obj list deletion be visible to other processors
244 before the atomic counter decrement is performed.
246 Otherwise, the counter could fall to zero, yet obj->active would still
247 be set, thus triggering the assertion in obj_destroy(). The error
248 sequence looks like this:
250 cpu 0 cpu 1
251 obj_poke() obj_timeout()
252 obj = obj_list_peek();
253 ... gains ref to obj, refcnt=2
254 obj_list_del(obj);
255 obj->active = 0 ...
256 ... visibility delayed ...
257 atomic_dec_and_test()
258 ... refcnt drops to 1 ...
259 atomic_dec_and_test()
260 ... refcount drops to 0 ...
261 obj_destroy()
262 BUG() triggers since obj->active
263 still seen as one
264 obj->active update visibility occurs
266 With the memory barrier semantics required of the atomic_t operations
267 which return values, the above sequence of memory visibility can never
268 happen. Specifically, in the above case the atomic_dec_and_test()
269 counter decrement would not become globally visible until the
270 obj->active update does.
272 As a historical note, 32-bit Sparc used to only allow usage of
273 24-bits of it's atomic_t type. This was because it used 8 bits
274 as a spinlock for SMP safety. Sparc32 lacked a "compare and swap"
275 type instruction. However, 32-bit Sparc has since been moved over
276 to a "hash table of spinlocks" scheme, that allows the full 32-bit
277 counter to be realized. Essentially, an array of spinlocks are
278 indexed into based upon the address of the atomic_t being operated
279 on, and that lock protects the atomic operation. Parisc uses the
280 same scheme.
282 Another note is that the atomic_t operations returning values are
283 extremely slow on an old 386.
285 We will now cover the atomic bitmask operations. You will find that
286 their SMP and memory barrier semantics are similar in shape and scope
287 to the atomic_t ops above.
289 Native atomic bit operations are defined to operate on objects aligned
290 to the size of an "unsigned long" C data type, and are least of that
291 size. The endianness of the bits within each "unsigned long" are the
292 native endianness of the cpu.
294 void set_bit(unsigned long nr, volatile unsigned long *addr);
295 void clear_bit(unsigned long nr, volatile unsigned long *addr);
296 void change_bit(unsigned long nr, volatile unsigned long *addr);
298 These routines set, clear, and change, respectively, the bit number
299 indicated by "nr" on the bit mask pointed to by "ADDR".
301 They must execute atomically, yet there are no implicit memory barrier
302 semantics required of these interfaces.
304 int test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
305 int test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
306 int test_and_change_bit(unsigned long nr, volatile unsigned long *addr);
308 Like the above, except that these routines return a boolean which
309 indicates whether the changed bit was set _BEFORE_ the atomic bit
310 operation.
312 WARNING! It is incredibly important that the value be a boolean,
313 ie. "0" or "1". Do not try to be fancy and save a few instructions by
314 declaring the above to return "long" and just returning something like
315 "old_val & mask" because that will not work.
317 For one thing, this return value gets truncated to int in many code
318 paths using these interfaces, so on 64-bit if the bit is set in the
319 upper 32-bits then testers will never see that.
321 One great example of where this problem crops up are the thread_info
322 flag operations. Routines such as test_and_set_ti_thread_flag() chop
323 the return value into an int. There are other places where things
324 like this occur as well.
326 These routines, like the atomic_t counter operations returning values,
327 require explicit memory barrier semantics around their execution. All
328 memory operations before the atomic bit operation call must be made
329 visible globally before the atomic bit operation is made visible.
330 Likewise, the atomic bit operation must be visible globally before any
331 subsequent memory operation is made visible. For example:
333 obj->dead = 1;
334 if (test_and_set_bit(0, &obj->flags))
335 /* ... */;
336 obj->killed = 1;
338 The implementation of test_and_set_bit() must guarantee that
339 "obj->dead = 1;" is visible to cpus before the atomic memory operation
340 done by test_and_set_bit() becomes visible. Likewise, the atomic
341 memory operation done by test_and_set_bit() must become visible before
342 "obj->killed = 1;" is visible.
344 Finally there is the basic operation:
346 int test_bit(unsigned long nr, __const__ volatile unsigned long *addr);
348 Which returns a boolean indicating if bit "nr" is set in the bitmask
349 pointed to by "addr".
351 If explicit memory barriers are required around clear_bit() (which
352 does not return a value, and thus does not need to provide memory
353 barrier semantics), two interfaces are provided:
355 void smp_mb__before_clear_bit(void);
356 void smp_mb__after_clear_bit(void);
358 They are used as follows, and are akin to their atomic_t operation
359 brothers:
361 /* All memory operations before this call will
362 * be globally visible before the clear_bit().
363 */
364 smp_mb__before_clear_bit();
365 clear_bit( ... );
367 /* The clear_bit() will be visible before all
368 * subsequent memory operations.
369 */
370 smp_mb__after_clear_bit();
372 Finally, there are non-atomic versions of the bitmask operations
373 provided. They are used in contexts where some other higher-level SMP
374 locking scheme is being used to protect the bitmask, and thus less
375 expensive non-atomic operations may be used in the implementation.
376 They have names similar to the above bitmask operation interfaces,
377 except that two underscores are prefixed to the interface name.
379 void __set_bit(unsigned long nr, volatile unsigned long *addr);
380 void __clear_bit(unsigned long nr, volatile unsigned long *addr);
381 void __change_bit(unsigned long nr, volatile unsigned long *addr);
382 int __test_and_set_bit(unsigned long nr, volatile unsigned long *addr);
383 int __test_and_clear_bit(unsigned long nr, volatile unsigned long *addr);
384 int __test_and_change_bit(unsigned long nr, volatile unsigned long *addr);
386 These non-atomic variants also do not require any special memory
387 barrier semantics.
389 The routines xchg() and cmpxchg() need the same exact memory barriers
390 as the atomic and bit operations returning values.
392 Spinlocks and rwlocks have memory barrier expectations as well.
393 The rule to follow is simple:
395 1) When acquiring a lock, the implementation must make it globally
396 visible before any subsequent memory operation.
398 2) When releasing a lock, the implementation must make it such that
399 all previous memory operations are globally visible before the
400 lock release.
402 Which finally brings us to _atomic_dec_and_lock(). There is an
403 architecture-neutral version implemented in lib/dec_and_lock.c,
404 but most platforms will wish to optimize this in assembler.
406 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock);
408 Atomically decrement the given counter, and if will drop to zero
409 atomically acquire the given spinlock and perform the decrement
410 of the counter to zero. If it does not drop to zero, do nothing
411 with the spinlock.
413 It is actually pretty simple to get the memory barrier correct.
414 Simply satisfy the spinlock grab requirements, which is make
415 sure the spinlock operation is globally visible before any
416 subsequent memory operation.
418 We can demonstrate this operation more clearly if we define
419 an abstract atomic operation:
421 long cas(long *mem, long old, long new);
423 "cas" stands for "compare and swap". It atomically:
425 1) Compares "old" with the value currently at "mem".
426 2) If they are equal, "new" is written to "mem".
427 3) Regardless, the current value at "mem" is returned.
429 As an example usage, here is what an atomic counter update
430 might look like:
432 void example_atomic_inc(long *counter)
433 {
434 long old, new, ret;
436 while (1) {
437 old = *counter;
438 new = old + 1;
440 ret = cas(counter, old, new);
441 if (ret == old)
442 break;
443 }
444 }
446 Let's use cas() in order to build a pseudo-C atomic_dec_and_lock():
448 int _atomic_dec_and_lock(atomic_t *atomic, spinlock_t *lock)
449 {
450 long old, new, ret;
451 int went_to_zero;
453 went_to_zero = 0;
454 while (1) {
455 old = atomic_read(atomic);
456 new = old - 1;
457 if (new == 0) {
458 went_to_zero = 1;
459 spin_lock(lock);
460 }
461 ret = cas(atomic, old, new);
462 if (ret == old)
463 break;
464 if (went_to_zero) {
465 spin_unlock(lock);
466 went_to_zero = 0;
467 }
468 }
470 return went_to_zero;
471 }
473 Now, as far as memory barriers go, as long as spin_lock()
474 strictly orders all subsequent memory operations (including
475 the cas()) with respect to itself, things will be fine.
477 Said another way, _atomic_dec_and_lock() must guarantee that
478 a counter dropping to zero is never made visible before the
479 spinlock being acquired.
481 Note that this also means that for the case where the counter
482 is not dropping to zero, there are no memory ordering
483 requirements.