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
  32 typedef struct {
  33         unsigned sequence;
  34         spinlock_t lock;
  35 } seqlock_t;
  36
  37 /*
  38  * These macros triggered gcc-3.x compile-time problems.  We think these are
  39  * OK now.  Be cautious.
  40  */
  41 #define __SEQLOCK_UNLOCKED(lockname) \
  42                  { 0, __SPIN_LOCK_UNLOCKED(lockname) }
  43
  44 #define seqlock_init(x)                                 \
  45         do {                                            \
  46                 (x)->sequence = 0;                      \
  47                 spin_lock_init(&(x)->lock);             \
  48         } while (0)
  49
  50 #define DEFINE_SEQLOCK(x) \
  51                 seqlock_t x = __SEQLOCK_UNLOCKED(x)
  52
  53 /* Lock out other writers and update the count.
  54  * Acts like a normal spin_lock/unlock.
  55  * Don't need preempt_disable() because that is in the spin_lock already.
  56  */
  57 static inline void write_seqlock(seqlock_t *sl)
  58 {
  59         spin_lock(&sl->lock);
  60         ++sl->sequence;
  61         smp_wmb();
  62 }
  63
  64 static inline void write_sequnlock(seqlock_t *sl)
  65 {
  66         smp_wmb();
  67         sl->sequence++;
  68         spin_unlock(&sl->lock);
  69 }
  70
  71 static inline int write_tryseqlock(seqlock_t *sl)
  72 {
  73         int ret = spin_trylock(&sl->lock);
  74
  75         if (ret) {
  76                 ++sl->sequence;
  77                 smp_wmb();
  78         }
  79         return ret;
  80 }
  81
  82 /* Start of read calculation -- fetch last complete writer token */
  83 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  84 {
  85         unsigned ret;
  86
  87 repeat:
  88         ret = ACCESS_ONCE(sl->sequence);
  89         if (unlikely(ret & 1)) {
  90                 cpu_relax();
  91                 goto repeat;
  92         }
  93         smp_rmb();
  94
  95         return ret;
  96 }
  97
  98 /*
  99  * Test if reader processed invalid data.
 100  *
 101  * If sequence value changed then writer changed data while in section.
 102  */
 103 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
 104 {
 105         smp_rmb();
 106
 107         return unlikely(sl->sequence != start);
 108 }
 109
 110
 111 /*
 112  * Version using sequence counter only.
 113  * This can be used when code has its own mutex protecting the
 114  * updating starting before the write_seqcountbeqin() and ending
 115  * after the write_seqcount_end().
 116  */
 117
 118 typedef struct seqcount {
 119         unsigned sequence;
 120 } seqcount_t;
 121
 122 #define SEQCNT_ZERO { 0 }
 123 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 124
 125 /**
 126  * __read_seqcount_begin - begin a seq-read critical section (without barrier)
 127  * @s: pointer to seqcount_t
 128  * Returns: count to be passed to read_seqcount_retry
 129  *
 130  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
 131  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 132  * provided before actually loading any of the variables that are to be
 133  * protected in this critical section.
 134  *
 135  * Use carefully, only in critical code, and comment how the barrier is
 136  * provided.
 137  */
 138 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
 139 {
 140         unsigned ret;
 141
 142 repeat:
 143         ret = s->sequence;
 144         if (unlikely(ret & 1)) {
 145                 cpu_relax();
 146                 goto repeat;
 147         }
 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  */
 160 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 161 {
 162         unsigned ret = __read_seqcount_begin(s);
 163         smp_rmb();
 164         return ret;
 165 }
 166
 167 /**
 168  * __read_seqcount_retry - end a seq-read critical section (without barrier)
 169  * @s: pointer to seqcount_t
 170  * @start: count, from read_seqcount_begin
 171  * Returns: 1 if retry is required, else 0
 172  *
 173  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 174  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 175  * provided before actually loading any of the variables that are to be
 176  * protected in this critical section.
 177  *
 178  * Use carefully, only in critical code, and comment how the barrier is
 179  * provided.
 180  */
 181 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
 182 {
 183         return unlikely(s->sequence != start);
 184 }
 185
 186 /**
 187  * read_seqcount_retry - end a seq-read critical section
 188  * @s: pointer to seqcount_t
 189  * @start: count, from read_seqcount_begin
 190  * Returns: 1 if retry is required, else 0
 191  *
 192  * read_seqcount_retry closes a read critical section of the given seqcount.
 193  * If the critical section was invalid, it must be ignored (and typically
 194  * retried).
 195  */
 196 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
 197 {
 198         smp_rmb();
 199
 200         return __read_seqcount_retry(s, start);
 201 }
 202
 203
 204 /*
 205  * Sequence counter only version assumes that callers are using their
 206  * own mutexing.
 207  */
 208 static inline void write_seqcount_begin(seqcount_t *s)
 209 {
 210         s->sequence++;
 211         smp_wmb();
 212 }
 213
 214 static inline void write_seqcount_end(seqcount_t *s)
 215 {
 216         smp_wmb();
 217         s->sequence++;
 218 }
 219
 220 /**
 221  * write_seqcount_barrier - invalidate in-progress read-side seq operations
 222  * @s: pointer to seqcount_t
 223  *
 224  * After write_seqcount_barrier, no read-side seq operations will complete
 225  * successfully and see data older than this.
 226  */
 227 static inline void write_seqcount_barrier(seqcount_t *s)
 228 {
 229         smp_wmb();
 230         s->sequence+=2;
 231 }
 232
 233 /*
 234  * Possible sw/hw IRQ protected versions of the interfaces.
 235  */
 236 #define write_seqlock_irqsave(lock, flags)                              \
 237         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 238 #define write_seqlock_irq(lock)                                         \
 239         do { local_irq_disable();   write_seqlock(lock); } while (0)
 240 #define write_seqlock_bh(lock)                                          \
 241         do { local_bh_disable();    write_seqlock(lock); } while (0)
 242
 243 #define write_sequnlock_irqrestore(lock, flags)                         \
 244         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 245 #define write_sequnlock_irq(lock)                                       \
 246         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 247 #define write_sequnlock_bh(lock)                                        \
 248         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 249
 250 #define read_seqbegin_irqsave(lock, flags)                              \
 251         ({ local_irq_save(flags);   read_seqbegin(lock); })
 252
 253 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 254         ({                                                              \
 255                 int ret = read_seqretry(lock, iv);                      \
 256                 local_irq_restore(flags);                               \
 257                 ret;                                                    \
 258         })
 259
 260 #endif /* __LINUX_SEQLOCK_H */