blob: cf87a24c0f92f1963081c93d7b0cce2356025358 [file] [log] [blame]
Linus Torvalds1da177e2005-04-16 15:20:36 -07001#ifndef __LINUX_SEQLOCK_H
2#define __LINUX_SEQLOCK_H
3/*
4 * Reader/writer consistent mechanism without starving writers. This type of
Robert P. J. Dayd08df602007-02-17 19:07:33 +01005 * lock for data where the reader wants a consistent set of information
Waiman Long1370e972013-09-12 10:55:34 -04006 * and is willing to retry if the information changes. There are two types
7 * of readers:
8 * 1. Sequence readers which never block a writer but they may have to retry
9 * if a writer is in progress by detecting change in sequence number.
10 * Writers do not wait for a sequence reader.
11 * 2. Locking readers which will wait if a writer or another locking reader
12 * is in progress. A locking reader in progress will also block a writer
13 * from going forward. Unlike the regular rwlock, the read lock here is
14 * exclusive so that only one locking reader can get it.
Linus Torvalds1da177e2005-04-16 15:20:36 -070015 *
Waiman Long1370e972013-09-12 10:55:34 -040016 * This is not as cache friendly as brlock. Also, this may not work well
Linus Torvalds1da177e2005-04-16 15:20:36 -070017 * for data that contains pointers, because any writer could
18 * invalidate a pointer that a reader was following.
19 *
Waiman Long1370e972013-09-12 10:55:34 -040020 * Expected non-blocking reader usage:
Linus Torvalds1da177e2005-04-16 15:20:36 -070021 * do {
22 * seq = read_seqbegin(&foo);
23 * ...
24 * } while (read_seqretry(&foo, seq));
25 *
26 *
27 * On non-SMP the spin locks disappear but the writer still needs
28 * to increment the sequence variables because an interrupt routine could
29 * change the state of the data.
30 *
31 * Based on x86_64 vsyscall gettimeofday
32 * by Keith Owens and Andrea Arcangeli
33 */
34
Linus Torvalds1da177e2005-04-16 15:20:36 -070035#include <linux/spinlock.h>
36#include <linux/preempt.h>
John Stultz1ca7d672013-10-07 15:51:59 -070037#include <linux/lockdep.h>
David Howells56a21052011-06-11 12:29:58 +010038#include <asm/processor.h>
Linus Torvalds1da177e2005-04-16 15:20:36 -070039
Linus Torvalds1da177e2005-04-16 15:20:36 -070040/*
41 * Version using sequence counter only.
42 * This can be used when code has its own mutex protecting the
43 * updating starting before the write_seqcountbeqin() and ending
44 * after the write_seqcount_end().
45 */
Linus Torvalds1da177e2005-04-16 15:20:36 -070046typedef struct seqcount {
47 unsigned sequence;
John Stultz1ca7d672013-10-07 15:51:59 -070048#ifdef CONFIG_DEBUG_LOCK_ALLOC
49 struct lockdep_map dep_map;
50#endif
Linus Torvalds1da177e2005-04-16 15:20:36 -070051} seqcount_t;
52
John Stultz1ca7d672013-10-07 15:51:59 -070053static inline void __seqcount_init(seqcount_t *s, const char *name,
54 struct lock_class_key *key)
55{
56 /*
57 * Make sure we are not reinitializing a held lock:
58 */
59 lockdep_init_map(&s->dep_map, name, key, 0);
60 s->sequence = 0;
61}
62
63#ifdef CONFIG_DEBUG_LOCK_ALLOC
64# define SEQCOUNT_DEP_MAP_INIT(lockname) \
65 .dep_map = { .name = #lockname } \
66
67# define seqcount_init(s) \
68 do { \
69 static struct lock_class_key __key; \
70 __seqcount_init((s), #s, &__key); \
71 } while (0)
72
73static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
74{
75 seqcount_t *l = (seqcount_t *)s;
76 unsigned long flags;
77
78 local_irq_save(flags);
79 seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
80 seqcount_release(&l->dep_map, 1, _RET_IP_);
81 local_irq_restore(flags);
82}
83
84#else
85# define SEQCOUNT_DEP_MAP_INIT(lockname)
86# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
87# define seqcount_lockdep_reader_access(x)
88#endif
89
90#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
91
Linus Torvalds1da177e2005-04-16 15:20:36 -070092
Nick Piggin3c22cd52011-01-07 17:49:51 +110093/**
94 * __read_seqcount_begin - begin a seq-read critical section (without barrier)
95 * @s: pointer to seqcount_t
96 * Returns: count to be passed to read_seqcount_retry
97 *
98 * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
99 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
100 * provided before actually loading any of the variables that are to be
101 * protected in this critical section.
102 *
103 * Use carefully, only in critical code, and comment how the barrier is
104 * provided.
105 */
106static inline unsigned __read_seqcount_begin(const seqcount_t *s)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700107{
Ingo Molnar88a411c2008-04-03 09:06:13 +0200108 unsigned ret;
109
110repeat:
Linus Torvalds2f624272012-05-04 14:46:02 -0700111 ret = ACCESS_ONCE(s->sequence);
Ingo Molnar88a411c2008-04-03 09:06:13 +0200112 if (unlikely(ret & 1)) {
113 cpu_relax();
114 goto repeat;
115 }
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116 return ret;
117}
118
Nick Piggin3c22cd52011-01-07 17:49:51 +1100119/**
John Stultz1ca7d672013-10-07 15:51:59 -0700120 * read_seqcount_begin_no_lockdep - start seq-read critical section w/o lockdep
121 * @s: pointer to seqcount_t
122 * Returns: count to be passed to read_seqcount_retry
123 *
124 * read_seqcount_begin_no_lockdep opens a read critical section of the given
125 * seqcount, but without any lockdep checking. Validity of the critical
126 * section is tested by checking read_seqcount_retry function.
127 */
128static inline unsigned read_seqcount_begin_no_lockdep(const seqcount_t *s)
129{
130 unsigned ret = __read_seqcount_begin(s);
131 smp_rmb();
132 return ret;
133}
134
135/**
Nick Piggin3c22cd52011-01-07 17:49:51 +1100136 * read_seqcount_begin - begin a seq-read critical section
137 * @s: pointer to seqcount_t
138 * Returns: count to be passed to read_seqcount_retry
139 *
140 * read_seqcount_begin opens a read critical section of the given seqcount.
141 * Validity of the critical section is tested by checking read_seqcount_retry
142 * function.
143 */
144static inline unsigned read_seqcount_begin(const seqcount_t *s)
145{
John Stultz1ca7d672013-10-07 15:51:59 -0700146 seqcount_lockdep_reader_access(s);
147 return read_seqcount_begin_no_lockdep(s);
Nick Piggin3c22cd52011-01-07 17:49:51 +1100148}
149
150/**
Linus Torvalds4f988f12012-05-04 15:13:54 -0700151 * raw_seqcount_begin - begin a seq-read critical section
152 * @s: pointer to seqcount_t
153 * Returns: count to be passed to read_seqcount_retry
154 *
155 * raw_seqcount_begin opens a read critical section of the given seqcount.
156 * Validity of the critical section is tested by checking read_seqcount_retry
157 * function.
158 *
159 * Unlike read_seqcount_begin(), this function will not wait for the count
160 * to stabilize. If a writer is active when we begin, we will fail the
161 * read_seqcount_retry() instead of stabilizing at the beginning of the
162 * critical section.
163 */
164static inline unsigned raw_seqcount_begin(const seqcount_t *s)
165{
166 unsigned ret = ACCESS_ONCE(s->sequence);
John Stultz1ca7d672013-10-07 15:51:59 -0700167
168 seqcount_lockdep_reader_access(s);
Linus Torvalds4f988f12012-05-04 15:13:54 -0700169 smp_rmb();
170 return ret & ~1;
171}
172
173/**
Nick Piggin3c22cd52011-01-07 17:49:51 +1100174 * __read_seqcount_retry - end a seq-read critical section (without barrier)
175 * @s: pointer to seqcount_t
176 * @start: count, from read_seqcount_begin
177 * Returns: 1 if retry is required, else 0
178 *
179 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
180 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
181 * provided before actually loading any of the variables that are to be
182 * protected in this critical section.
183 *
184 * Use carefully, only in critical code, and comment how the barrier is
185 * provided.
186 */
187static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
188{
189 return unlikely(s->sequence != start);
190}
191
192/**
193 * read_seqcount_retry - end a seq-read critical section
194 * @s: pointer to seqcount_t
195 * @start: count, from read_seqcount_begin
196 * Returns: 1 if retry is required, else 0
197 *
198 * read_seqcount_retry closes a read critical section of the given seqcount.
199 * If the critical section was invalid, it must be ignored (and typically
200 * retried).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700201 */
Ingo Molnar88a411c2008-04-03 09:06:13 +0200202static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700203{
204 smp_rmb();
Nick Piggin3c22cd52011-01-07 17:49:51 +1100205 return __read_seqcount_retry(s, start);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700206}
207
208
209/*
210 * Sequence counter only version assumes that callers are using their
211 * own mutexing.
212 */
John Stultz1ca7d672013-10-07 15:51:59 -0700213static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700214{
215 s->sequence++;
216 smp_wmb();
John Stultz1ca7d672013-10-07 15:51:59 -0700217 seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
218}
219
220static inline void write_seqcount_begin(seqcount_t *s)
221{
222 write_seqcount_begin_nested(s, 0);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700223}
224
225static inline void write_seqcount_end(seqcount_t *s)
226{
John Stultz1ca7d672013-10-07 15:51:59 -0700227 seqcount_release(&s->dep_map, 1, _RET_IP_);
Linus Torvalds1da177e2005-04-16 15:20:36 -0700228 smp_wmb();
229 s->sequence++;
230}
231
Nick Piggin3c22cd52011-01-07 17:49:51 +1100232/**
233 * write_seqcount_barrier - invalidate in-progress read-side seq operations
234 * @s: pointer to seqcount_t
235 *
236 * After write_seqcount_barrier, no read-side seq operations will complete
237 * successfully and see data older than this.
238 */
239static inline void write_seqcount_barrier(seqcount_t *s)
240{
241 smp_wmb();
242 s->sequence+=2;
243}
244
Thomas Gleixner6617fec2011-07-16 18:40:26 +0200245typedef struct {
246 struct seqcount seqcount;
247 spinlock_t lock;
248} seqlock_t;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700249
Thomas Gleixner6617fec2011-07-16 18:40:26 +0200250/*
251 * These macros triggered gcc-3.x compile-time problems. We think these are
252 * OK now. Be cautious.
253 */
254#define __SEQLOCK_UNLOCKED(lockname) \
255 { \
John Stultz1ca7d672013-10-07 15:51:59 -0700256 .seqcount = SEQCNT_ZERO(lockname), \
Thomas Gleixner6617fec2011-07-16 18:40:26 +0200257 .lock = __SPIN_LOCK_UNLOCKED(lockname) \
258 }
259
260#define seqlock_init(x) \
261 do { \
262 seqcount_init(&(x)->seqcount); \
263 spin_lock_init(&(x)->lock); \
264 } while (0)
265
266#define DEFINE_SEQLOCK(x) \
267 seqlock_t x = __SEQLOCK_UNLOCKED(x)
268
269/*
270 * Read side functions for starting and finalizing a read side section.
271 */
272static inline unsigned read_seqbegin(const seqlock_t *sl)
273{
274 return read_seqcount_begin(&sl->seqcount);
275}
276
277static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
278{
279 return read_seqcount_retry(&sl->seqcount, start);
280}
281
282/*
283 * Lock out other writers and update the count.
284 * Acts like a normal spin_lock/unlock.
285 * Don't need preempt_disable() because that is in the spin_lock already.
286 */
287static inline void write_seqlock(seqlock_t *sl)
288{
289 spin_lock(&sl->lock);
290 write_seqcount_begin(&sl->seqcount);
291}
292
293static inline void write_sequnlock(seqlock_t *sl)
294{
295 write_seqcount_end(&sl->seqcount);
296 spin_unlock(&sl->lock);
297}
298
299static inline void write_seqlock_bh(seqlock_t *sl)
300{
301 spin_lock_bh(&sl->lock);
302 write_seqcount_begin(&sl->seqcount);
303}
304
305static inline void write_sequnlock_bh(seqlock_t *sl)
306{
307 write_seqcount_end(&sl->seqcount);
308 spin_unlock_bh(&sl->lock);
309}
310
311static inline void write_seqlock_irq(seqlock_t *sl)
312{
313 spin_lock_irq(&sl->lock);
314 write_seqcount_begin(&sl->seqcount);
315}
316
317static inline void write_sequnlock_irq(seqlock_t *sl)
318{
319 write_seqcount_end(&sl->seqcount);
320 spin_unlock_irq(&sl->lock);
321}
322
323static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
324{
325 unsigned long flags;
326
327 spin_lock_irqsave(&sl->lock, flags);
328 write_seqcount_begin(&sl->seqcount);
329 return flags;
330}
331
332#define write_seqlock_irqsave(lock, flags) \
333 do { flags = __write_seqlock_irqsave(lock); } while (0)
334
335static inline void
336write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
337{
338 write_seqcount_end(&sl->seqcount);
339 spin_unlock_irqrestore(&sl->lock, flags);
340}
Linus Torvalds1da177e2005-04-16 15:20:36 -0700341
Waiman Long1370e972013-09-12 10:55:34 -0400342/*
343 * A locking reader exclusively locks out other writers and locking readers,
344 * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
345 * Don't need preempt_disable() because that is in the spin_lock already.
346 */
347static inline void read_seqlock_excl(seqlock_t *sl)
348{
349 spin_lock(&sl->lock);
350}
351
352static inline void read_sequnlock_excl(seqlock_t *sl)
353{
354 spin_unlock(&sl->lock);
355}
356
Al Viro2bc74fe2013-10-25 16:39:14 -0400357/**
358 * read_seqbegin_or_lock - begin a sequence number check or locking block
359 * @lock: sequence lock
360 * @seq : sequence number to be checked
361 *
362 * First try it once optimistically without taking the lock. If that fails,
363 * take the lock. The sequence number is also used as a marker for deciding
364 * whether to be a reader (even) or writer (odd).
365 * N.B. seq must be initialized to an even number to begin with.
366 */
367static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
368{
369 if (!(*seq & 1)) /* Even */
370 *seq = read_seqbegin(lock);
371 else /* Odd */
372 read_seqlock_excl(lock);
373}
374
375static inline int need_seqretry(seqlock_t *lock, int seq)
376{
377 return !(seq & 1) && read_seqretry(lock, seq);
378}
379
380static inline void done_seqretry(seqlock_t *lock, int seq)
381{
382 if (seq & 1)
383 read_sequnlock_excl(lock);
384}
385
Waiman Long1370e972013-09-12 10:55:34 -0400386static inline void read_seqlock_excl_bh(seqlock_t *sl)
387{
388 spin_lock_bh(&sl->lock);
389}
390
391static inline void read_sequnlock_excl_bh(seqlock_t *sl)
392{
393 spin_unlock_bh(&sl->lock);
394}
395
396static inline void read_seqlock_excl_irq(seqlock_t *sl)
397{
398 spin_lock_irq(&sl->lock);
399}
400
401static inline void read_sequnlock_excl_irq(seqlock_t *sl)
402{
403 spin_unlock_irq(&sl->lock);
404}
405
406static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
407{
408 unsigned long flags;
409
410 spin_lock_irqsave(&sl->lock, flags);
411 return flags;
412}
413
414#define read_seqlock_excl_irqsave(lock, flags) \
415 do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
416
417static inline void
418read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
419{
420 spin_unlock_irqrestore(&sl->lock, flags);
421}
422
Linus Torvalds1da177e2005-04-16 15:20:36 -0700423#endif /* __LINUX_SEQLOCK_H */