include/linux/seqlock.h

   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.  Readers never
   7  * block but they may have to retry if a writer is in
   8  * progress. Writers do not wait for readers.
   9  *
  10  * This is not as cache friendly as brlock. Also, this will not work
  11  * for data that contains pointers, because any writer could
  12  * invalidate a pointer that a reader was following.
  13  *
  14  * Expected reader usage:
  15  *      do {
  16  *          seq = read_seqbegin(&foo);
  17  *      ...
  18  *      } while (read_seqretry(&foo, seq));
  19  *
  20  *
  21  * On non-SMP the spin locks disappear but the writer still needs
  22  * to increment the sequence variables because an interrupt routine could
  23  * change the state of the data.
  24  *
  25  * Based on x86_64 vsyscall gettimeofday
  26  * by Keith Owens and Andrea Arcangeli
  27  */
  28
  29 #include <linux/spinlock.h>
  30 #include <linux/preempt.h>
  31 #include <asm/processor.h>
  32
  33 typedef struct {
  34         unsigned sequence;
  35         spinlock_t lock;
  36 } seqlock_t;
  37
  38 /*
  39  * These macros triggered gcc-3.x compile-time problems.  We think these are
  40  * OK now.  Be cautious.
  41  */
  42 #define __SEQLOCK_UNLOCKED(lockname) \
  43                  { 0, __SPIN_LOCK_UNLOCKED(lockname) }
  44
  45 #define seqlock_init(x)                                 \
  46         do {                                            \
  47                 (x)->sequence = 0;                      \
  48                 spin_lock_init(&(x)->lock);             \
  49         } while (0)
  50
  51 #define DEFINE_SEQLOCK(x) \
  52                 seqlock_t x = __SEQLOCK_UNLOCKED(x)
  53
  54 /* Lock out other writers and update the count.
  55  * Acts like a normal spin_lock/unlock.
  56  * Don't need preempt_disable() because that is in the spin_lock already.
  57  */
  58 static inline void write_seqlock(seqlock_t *sl)
  59 {
  60         spin_lock(&sl->lock);
  61         ++sl->sequence;
  62         smp_wmb();
  63 }
  64
  65 static inline void write_sequnlock(seqlock_t *sl)
  66 {
  67         smp_wmb();
  68         sl->sequence++;
  69         spin_unlock(&sl->lock);
  70 }
  71
  72 static inline int write_tryseqlock(seqlock_t *sl)
  73 {
  74         int ret = spin_trylock(&sl->lock);
  75
  76         if (ret) {
  77                 ++sl->sequence;
  78                 smp_wmb();
  79         }
  80         return ret;
  81 }
  82
  83 /* Start of read calculation -- fetch last complete writer token */
  84 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  85 {
  86         unsigned ret;
  87
  88 repeat:
  89         ret = ACCESS_ONCE(sl->sequence);
  90         if (unlikely(ret & 1)) {
  91                 cpu_relax();
  92                 goto repeat;
  93         }
  94         smp_rmb();
  95
  96         return ret;
  97 }
  98
  99 /*
 100  * Test if reader processed invalid data.
 101  *
 102  * If sequence value changed then writer changed data while in section.
 103  */
 104 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
 105 {
 106         smp_rmb();
 107
 108         return unlikely(sl->sequence != start);
 109 }
 110
 111
 112 /*
 113  * Version using sequence counter only.
 114  * This can be used when code has its own mutex protecting the
 115  * updating starting before the write_seqcountbeqin() and ending
 116  * after the write_seqcount_end().
 117  */
 118
 119 typedef struct seqcount {
 120         unsigned sequence;
 121 } seqcount_t;
 122
 123 #define SEQCNT_ZERO { 0 }
 124 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 125
 126 /**
 127  * __read_seqcount_begin - begin a seq-read critical section (without barrier)
 128  * @s: pointer to seqcount_t
 129  * Returns: count to be passed to read_seqcount_retry
 130  *
 131  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
 132  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 133  * provided before actually loading any of the variables that are to be
 134  * protected in this critical section.
 135  *
 136  * Use carefully, only in critical code, and comment how the barrier is
 137  * provided.
 138  */
 139 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
 140 {
 141         unsigned ret;
 142
 143 repeat:
 144         ret = ACCESS_ONCE(s->sequence);
 145         if (unlikely(ret & 1)) {
 146                 cpu_relax();
 147                 goto repeat;
 148         }
 149         return ret;
 150 }
 151
 152 /**
 153  * read_seqcount_begin - begin a seq-read critical section
 154  * @s: pointer to seqcount_t
 155  * Returns: count to be passed to read_seqcount_retry
 156  *
 157  * read_seqcount_begin opens a read critical section of the given seqcount.
 158  * Validity of the critical section is tested by checking read_seqcount_retry
 159  * function.
 160  */
 161 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 162 {
 163         unsigned ret = __read_seqcount_begin(s);
 164         smp_rmb();
 165         return ret;
 166 }
 167
 168 /**
 169  * raw_seqcount_begin - begin a seq-read critical section
 170  * @s: pointer to seqcount_t
 171  * Returns: count to be passed to read_seqcount_retry
 172  *
 173  * raw_seqcount_begin opens a read critical section of the given seqcount.
 174  * Validity of the critical section is tested by checking read_seqcount_retry
 175  * function.
 176  *
 177  * Unlike read_seqcount_begin(), this function will not wait for the count
 178  * to stabilize. If a writer is active when we begin, we will fail the
 179  * read_seqcount_retry() instead of stabilizing at the beginning of the
 180  * critical section.
 181  */
 182 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
 183 {
 184         unsigned ret = ACCESS_ONCE(s->sequence);
 185         smp_rmb();
 186         return ret & ~1;
 187 }
 188
 189 /**
 190  * __read_seqcount_retry - end a seq-read critical section (without barrier)
 191  * @s: pointer to seqcount_t
 192  * @start: count, from read_seqcount_begin
 193  * Returns: 1 if retry is required, else 0
 194  *
 195  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 196  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 197  * provided before actually loading any of the variables that are to be
 198  * protected in this critical section.
 199  *
 200  * Use carefully, only in critical code, and comment how the barrier is
 201  * provided.
 202  */
 203 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
 204 {
 205         return unlikely(s->sequence != start);
 206 }
 207
 208 /**
 209  * read_seqcount_retry - end a seq-read critical section
 210  * @s: pointer to seqcount_t
 211  * @start: count, from read_seqcount_begin
 212  * Returns: 1 if retry is required, else 0
 213  *
 214  * read_seqcount_retry closes a read critical section of the given seqcount.
 215  * If the critical section was invalid, it must be ignored (and typically
 216  * retried).
 217  */
 218 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
 219 {
 220         smp_rmb();
 221
 222         return __read_seqcount_retry(s, start);
 223 }
 224
 225
 226 /*
 227  * Sequence counter only version assumes that callers are using their
 228  * own mutexing.
 229  */
 230 static inline void write_seqcount_begin(seqcount_t *s)
 231 {
 232         s->sequence++;
 233         smp_wmb();
 234 }
 235
 236 static inline void write_seqcount_end(seqcount_t *s)
 237 {
 238         smp_wmb();
 239         s->sequence++;
 240 }
 241
 242 /**
 243  * write_seqcount_barrier - invalidate in-progress read-side seq operations
 244  * @s: pointer to seqcount_t
 245  *
 246  * After write_seqcount_barrier, no read-side seq operations will complete
 247  * successfully and see data older than this.
 248  */
 249 static inline void write_seqcount_barrier(seqcount_t *s)
 250 {
 251         smp_wmb();
 252         s->sequence+=2;
 253 }
 254
 255 /*
 256  * Possible sw/hw IRQ protected versions of the interfaces.
 257  */
 258 #define write_seqlock_irqsave(lock, flags)                              \
 259         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 260 #define write_seqlock_irq(lock)                                         \
 261         do { local_irq_disable();   write_seqlock(lock); } while (0)
 262 #define write_seqlock_bh(lock)                                          \
 263         do { local_bh_disable();    write_seqlock(lock); } while (0)
 264
 265 #define write_sequnlock_irqrestore(lock, flags)                         \
 266         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 267 #define write_sequnlock_irq(lock)                                       \
 268         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 269 #define write_sequnlock_bh(lock)                                        \
 270         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 271
 272 #define read_seqbegin_irqsave(lock, flags)                              \
 273         ({ local_irq_save(flags);   read_seqbegin(lock); })
 274
 275 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 276         ({                                                              \
 277                 int ret = read_seqretry(lock, iv);                      \
 278                 local_irq_restore(flags);                               \
 279                 ret;                                                    \
 280         })
 281
 282 #endif /* __LINUX_SEQLOCK_H */