1#ifndef __LINUX_SEQLOCK_H 2#define __LINUX_SEQLOCK_H 3/* 4 * Reader/writer consistent mechanism without starving writers. This type of 5 * lock for data where the reader wants a consistent set of information 6 * 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. 15 * 16 * This is not as cache friendly as brlock. Also, this may not work well 17 * for data that contains pointers, because any writer could 18 * invalidate a pointer that a reader was following. 19 * 20 * Expected non-blocking reader usage: 21 * 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 35#include <linux/spinlock.h> 36#include <linux/preempt.h> 37#include <linux/lockdep.h> 38#include <asm/processor.h> 39 40/* 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 */ 46typedef struct seqcount { 47 unsigned sequence; 48#ifdef CONFIG_DEBUG_LOCK_ALLOC 49 struct lockdep_map dep_map; 50#endif 51} seqcount_t; 52 53static 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 92 93/** 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) 107{ 108 unsigned ret; 109 110repeat: 111 ret = READ_ONCE(s->sequence); 112 if (unlikely(ret & 1)) { 113 cpu_relax(); 114 goto repeat; 115 } 116 return ret; 117} 118 119/** 120 * raw_read_seqcount - Read the raw seqcount 121 * @s: pointer to seqcount_t 122 * Returns: count to be passed to read_seqcount_retry 123 * 124 * raw_read_seqcount opens a read critical section of the given 125 * seqcount without any lockdep checking and without checking or 126 * masking the LSB. Calling code is responsible for handling that. 127 */ 128static inline unsigned raw_read_seqcount(const seqcount_t *s) 129{ 130 unsigned ret = READ_ONCE(s->sequence); 131 smp_rmb(); 132 return ret; 133} 134 135/** 136 * raw_read_seqcount_begin - start seq-read critical section w/o lockdep 137 * @s: pointer to seqcount_t 138 * Returns: count to be passed to read_seqcount_retry 139 * 140 * raw_read_seqcount_begin opens a read critical section of the given 141 * seqcount, but without any lockdep checking. Validity of the critical 142 * section is tested by checking read_seqcount_retry function. 143 */ 144static inline unsigned raw_read_seqcount_begin(const seqcount_t *s) 145{ 146 unsigned ret = __read_seqcount_begin(s); 147 smp_rmb(); 148 return ret; 149} 150 151/** 152 * read_seqcount_begin - begin a seq-read critical section 153 * @s: pointer to seqcount_t 154 * Returns: count to be passed to read_seqcount_retry 155 * 156 * read_seqcount_begin opens a read critical section of the given seqcount. 157 * Validity of the critical section is tested by checking read_seqcount_retry 158 * function. 159 */ 160static inline unsigned read_seqcount_begin(const seqcount_t *s) 161{ 162 seqcount_lockdep_reader_access(s); 163 return raw_read_seqcount_begin(s); 164} 165 166/** 167 * raw_seqcount_begin - begin a seq-read critical section 168 * @s: pointer to seqcount_t 169 * Returns: count to be passed to read_seqcount_retry 170 * 171 * raw_seqcount_begin opens a read critical section of the given seqcount. 172 * Validity of the critical section is tested by checking read_seqcount_retry 173 * function. 174 * 175 * Unlike read_seqcount_begin(), this function will not wait for the count 176 * to stabilize. If a writer is active when we begin, we will fail the 177 * read_seqcount_retry() instead of stabilizing at the beginning of the 178 * critical section. 179 */ 180static inline unsigned raw_seqcount_begin(const seqcount_t *s) 181{ 182 unsigned ret = READ_ONCE(s->sequence); 183 smp_rmb(); 184 return ret & ~1; 185} 186 187/** 188 * __read_seqcount_retry - end a seq-read critical section (without barrier) 189 * @s: pointer to seqcount_t 190 * @start: count, from read_seqcount_begin 191 * Returns: 1 if retry is required, else 0 192 * 193 * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb() 194 * barrier. Callers should ensure that smp_rmb() or equivalent ordering is 195 * provided before actually loading any of the variables that are to be 196 * protected in this critical section. 197 * 198 * Use carefully, only in critical code, and comment how the barrier is 199 * provided. 200 */ 201static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start) 202{ 203 return unlikely(s->sequence != start); 204} 205 206/** 207 * read_seqcount_retry - end a seq-read critical section 208 * @s: pointer to seqcount_t 209 * @start: count, from read_seqcount_begin 210 * Returns: 1 if retry is required, else 0 211 * 212 * read_seqcount_retry closes a read critical section of the given seqcount. 213 * If the critical section was invalid, it must be ignored (and typically 214 * retried). 215 */ 216static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) 217{ 218 smp_rmb(); 219 return __read_seqcount_retry(s, start); 220} 221 222 223 224static inline void raw_write_seqcount_begin(seqcount_t *s) 225{ 226 s->sequence++; 227 smp_wmb(); 228} 229 230static inline void raw_write_seqcount_end(seqcount_t *s) 231{ 232 smp_wmb(); 233 s->sequence++; 234} 235 236/* 237 * raw_write_seqcount_latch - redirect readers to even/odd copy 238 * @s: pointer to seqcount_t 239 */ 240static inline void raw_write_seqcount_latch(seqcount_t *s) 241{ 242 smp_wmb(); /* prior stores before incrementing "sequence" */ 243 s->sequence++; 244 smp_wmb(); /* increment "sequence" before following stores */ 245} 246 247/* 248 * Sequence counter only version assumes that callers are using their 249 * own mutexing. 250 */ 251static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass) 252{ 253 raw_write_seqcount_begin(s); 254 seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_); 255} 256 257static inline void write_seqcount_begin(seqcount_t *s) 258{ 259 write_seqcount_begin_nested(s, 0); 260} 261 262static inline void write_seqcount_end(seqcount_t *s) 263{ 264 seqcount_release(&s->dep_map, 1, _RET_IP_); 265 raw_write_seqcount_end(s); 266} 267 268/** 269 * write_seqcount_invalidate - invalidate in-progress read-side seq operations 270 * @s: pointer to seqcount_t 271 * 272 * After write_seqcount_invalidate, no read-side seq operations will complete 273 * successfully and see data older than this. 274 */ 275static inline void write_seqcount_invalidate(seqcount_t *s) 276{ 277 smp_wmb(); 278 s->sequence+=2; 279} 280 281typedef struct { 282 struct seqcount seqcount; 283 spinlock_t lock; 284} seqlock_t; 285 286/* 287 * These macros triggered gcc-3.x compile-time problems. We think these are 288 * OK now. Be cautious. 289 */ 290#define __SEQLOCK_UNLOCKED(lockname) \ 291 { \ 292 .seqcount = SEQCNT_ZERO(lockname), \ 293 .lock = __SPIN_LOCK_UNLOCKED(lockname) \ 294 } 295 296#define seqlock_init(x) \ 297 do { \ 298 seqcount_init(&(x)->seqcount); \ 299 spin_lock_init(&(x)->lock); \ 300 } while (0) 301 302#define DEFINE_SEQLOCK(x) \ 303 seqlock_t x = __SEQLOCK_UNLOCKED(x) 304 305/* 306 * Read side functions for starting and finalizing a read side section. 307 */ 308static inline unsigned read_seqbegin(const seqlock_t *sl) 309{ 310 return read_seqcount_begin(&sl->seqcount); 311} 312 313static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) 314{ 315 return read_seqcount_retry(&sl->seqcount, start); 316} 317 318/* 319 * Lock out other writers and update the count. 320 * Acts like a normal spin_lock/unlock. 321 * Don't need preempt_disable() because that is in the spin_lock already. 322 */ 323static inline void write_seqlock(seqlock_t *sl) 324{ 325 spin_lock(&sl->lock); 326 write_seqcount_begin(&sl->seqcount); 327} 328 329static inline void write_sequnlock(seqlock_t *sl) 330{ 331 write_seqcount_end(&sl->seqcount); 332 spin_unlock(&sl->lock); 333} 334 335static inline void write_seqlock_bh(seqlock_t *sl) 336{ 337 spin_lock_bh(&sl->lock); 338 write_seqcount_begin(&sl->seqcount); 339} 340 341static inline void write_sequnlock_bh(seqlock_t *sl) 342{ 343 write_seqcount_end(&sl->seqcount); 344 spin_unlock_bh(&sl->lock); 345} 346 347static inline void write_seqlock_irq(seqlock_t *sl) 348{ 349 spin_lock_irq(&sl->lock); 350 write_seqcount_begin(&sl->seqcount); 351} 352 353static inline void write_sequnlock_irq(seqlock_t *sl) 354{ 355 write_seqcount_end(&sl->seqcount); 356 spin_unlock_irq(&sl->lock); 357} 358 359static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl) 360{ 361 unsigned long flags; 362 363 spin_lock_irqsave(&sl->lock, flags); 364 write_seqcount_begin(&sl->seqcount); 365 return flags; 366} 367 368#define write_seqlock_irqsave(lock, flags) \ 369 do { flags = __write_seqlock_irqsave(lock); } while (0) 370 371static inline void 372write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags) 373{ 374 write_seqcount_end(&sl->seqcount); 375 spin_unlock_irqrestore(&sl->lock, flags); 376} 377 378/* 379 * A locking reader exclusively locks out other writers and locking readers, 380 * but doesn't update the sequence number. Acts like a normal spin_lock/unlock. 381 * Don't need preempt_disable() because that is in the spin_lock already. 382 */ 383static inline void read_seqlock_excl(seqlock_t *sl) 384{ 385 spin_lock(&sl->lock); 386} 387 388static inline void read_sequnlock_excl(seqlock_t *sl) 389{ 390 spin_unlock(&sl->lock); 391} 392 393/** 394 * read_seqbegin_or_lock - begin a sequence number check or locking block 395 * @lock: sequence lock 396 * @seq : sequence number to be checked 397 * 398 * First try it once optimistically without taking the lock. If that fails, 399 * take the lock. The sequence number is also used as a marker for deciding 400 * whether to be a reader (even) or writer (odd). 401 * N.B. seq must be initialized to an even number to begin with. 402 */ 403static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq) 404{ 405 if (!(*seq & 1)) /* Even */ 406 *seq = read_seqbegin(lock); 407 else /* Odd */ 408 read_seqlock_excl(lock); 409} 410 411static inline int need_seqretry(seqlock_t *lock, int seq) 412{ 413 return !(seq & 1) && read_seqretry(lock, seq); 414} 415 416static inline void done_seqretry(seqlock_t *lock, int seq) 417{ 418 if (seq & 1) 419 read_sequnlock_excl(lock); 420} 421 422static inline void read_seqlock_excl_bh(seqlock_t *sl) 423{ 424 spin_lock_bh(&sl->lock); 425} 426 427static inline void read_sequnlock_excl_bh(seqlock_t *sl) 428{ 429 spin_unlock_bh(&sl->lock); 430} 431 432static inline void read_seqlock_excl_irq(seqlock_t *sl) 433{ 434 spin_lock_irq(&sl->lock); 435} 436 437static inline void read_sequnlock_excl_irq(seqlock_t *sl) 438{ 439 spin_unlock_irq(&sl->lock); 440} 441 442static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl) 443{ 444 unsigned long flags; 445 446 spin_lock_irqsave(&sl->lock, flags); 447 return flags; 448} 449 450#define read_seqlock_excl_irqsave(lock, flags) \ 451 do { flags = __read_seqlock_excl_irqsave(lock); } while (0) 452 453static inline void 454read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags) 455{ 456 spin_unlock_irqrestore(&sl->lock, flags); 457} 458 459static inline unsigned long 460read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq) 461{ 462 unsigned long flags = 0; 463 464 if (!(*seq & 1)) /* Even */ 465 *seq = read_seqbegin(lock); 466 else /* Odd */ 467 read_seqlock_excl_irqsave(lock, flags); 468 469 return flags; 470} 471 472static inline void 473done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags) 474{ 475 if (seq & 1) 476 read_sequnlock_excl_irqrestore(lock, flags); 477} 478#endif /* __LINUX_SEQLOCK_H */ 479