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_UNLOCKED \
  45                  __SEQLOCK_UNLOCKED(old_style_seqlock_init)
  46
  47 #define seqlock_init(x)                                 \
  48         do {                                            \
  49                 (x)->sequence = 0;                      \
  50                 spin_lock_init(&(x)->lock);             \
  51         } while (0)
  52
  53 #define DEFINE_SEQLOCK(x) \
  54                 seqlock_t x = __SEQLOCK_UNLOCKED(x)
  55
  56 /* Lock out other writers and update the count.
  57  * Acts like a normal spin_lock/unlock.
  58  * Don't need preempt_disable() because that is in the spin_lock already.
  59  */
  60 static inline void write_seqlock(seqlock_t *sl)
  61 {
  62         spin_lock(&sl->lock);
  63         ++sl->sequence;
  64         smp_wmb();
  65 }
  66
  67 static inline void write_sequnlock(seqlock_t *sl)
  68 {
  69         smp_wmb();
  70         sl->sequence++;
  71         spin_unlock(&sl->lock);
  72 }
  73
  74 static inline int write_tryseqlock(seqlock_t *sl)
  75 {
  76         int ret = spin_trylock(&sl->lock);
  77
  78         if (ret) {
  79                 ++sl->sequence;
  80                 smp_wmb();
  81         }
  82         return ret;
  83 }
  84
  85 /* Start of read calculation -- fetch last complete writer token */
  86 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  87 {
  88         unsigned ret;
  89
  90 repeat:
  91         ret = sl->sequence;
  92         smp_rmb();
  93         if (unlikely(ret & 1)) {
  94                 cpu_relax();
  95                 goto repeat;
  96         }
  97
  98         return ret;
  99 }
 100
 101 /*
 102  * Test if reader processed invalid data.
 103  *
 104  * If sequence value changed then writer changed data while in section.
 105  */
 106 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
 107 {
 108         smp_rmb();
 109
 110         return unlikely(sl->sequence != start);
 111 }
 112
 113
 114 /*
 115  * Version using sequence counter only.
 116  * This can be used when code has its own mutex protecting the
 117  * updating starting before the write_seqcountbeqin() and ending
 118  * after the write_seqcount_end().
 119  */
 120
 121 typedef struct seqcount {
 122         unsigned sequence;
 123 } seqcount_t;
 124
 125 #define SEQCNT_ZERO { 0 }
 126 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 127
 128 /**
 129  * __read_seqcount_begin - begin a seq-read critical section (without barrier)
 130  * @s: pointer to seqcount_t
 131  * Returns: count to be passed to read_seqcount_retry
 132  *
 133  * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
 134  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 135  * provided before actually loading any of the variables that are to be
 136  * protected in this critical section.
 137  *
 138  * Use carefully, only in critical code, and comment how the barrier is
 139  * provided.
 140  */
 141 static inline unsigned __read_seqcount_begin(const seqcount_t *s)
 142 {
 143         unsigned ret;
 144
 145 repeat:
 146         ret = s->sequence;
 147         if (unlikely(ret & 1)) {
 148                 cpu_relax();
 149                 goto repeat;
 150         }
 151         return ret;
 152 }
 153
 154 /**
 155  * read_seqcount_begin - begin a seq-read critical section
 156  * @s: pointer to seqcount_t
 157  * Returns: count to be passed to read_seqcount_retry
 158  *
 159  * read_seqcount_begin opens a read critical section of the given seqcount.
 160  * Validity of the critical section is tested by checking read_seqcount_retry
 161  * function.
 162  */
 163 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 164 {
 165         unsigned ret = __read_seqcount_begin(s);
 166         smp_rmb();
 167         return ret;
 168 }
 169
 170 /**
 171  * __read_seqcount_retry - end a seq-read critical section (without barrier)
 172  * @s: pointer to seqcount_t
 173  * @start: count, from read_seqcount_begin
 174  * Returns: 1 if retry is required, else 0
 175  *
 176  * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
 177  * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
 178  * provided before actually loading any of the variables that are to be
 179  * protected in this critical section.
 180  *
 181  * Use carefully, only in critical code, and comment how the barrier is
 182  * provided.
 183  */
 184 static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
 185 {
 186         return unlikely(s->sequence != start);
 187 }
 188
 189 /**
 190  * read_seqcount_retry - end a seq-read critical section
 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 closes a read critical section of the given seqcount.
 196  * If the critical section was invalid, it must be ignored (and typically
 197  * retried).
 198  */
 199 static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
 200 {
 201         smp_rmb();
 202
 203         return __read_seqcount_retry(s, start);
 204 }
 205
 206
 207 /*
 208  * Sequence counter only version assumes that callers are using their
 209  * own mutexing.
 210  */
 211 static inline void write_seqcount_begin(seqcount_t *s)
 212 {
 213         s->sequence++;
 214         smp_wmb();
 215 }
 216
 217 static inline void write_seqcount_end(seqcount_t *s)
 218 {
 219         smp_wmb();
 220         s->sequence++;
 221 }
 222
 223 /**
 224  * write_seqcount_barrier - invalidate in-progress read-side seq operations
 225  * @s: pointer to seqcount_t
 226  *
 227  * After write_seqcount_barrier, no read-side seq operations will complete
 228  * successfully and see data older than this.
 229  */
 230 static inline void write_seqcount_barrier(seqcount_t *s)
 231 {
 232         smp_wmb();
 233         s->sequence+=2;
 234 }
 235
 236 /*
 237  * Possible sw/hw IRQ protected versions of the interfaces.
 238  */
 239 #define write_seqlock_irqsave(lock, flags)                              \
 240         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 241 #define write_seqlock_irq(lock)                                         \
 242         do { local_irq_disable();   write_seqlock(lock); } while (0)
 243 #define write_seqlock_bh(lock)                                          \
 244         do { local_bh_disable();    write_seqlock(lock); } while (0)
 245
 246 #define write_sequnlock_irqrestore(lock, flags)                         \
 247         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 248 #define write_sequnlock_irq(lock)                                       \
 249         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 250 #define write_sequnlock_bh(lock)                                        \
 251         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 252
 253 #define read_seqbegin_irqsave(lock, flags)                              \
 254         ({ local_irq_save(flags);   read_seqbegin(lock); })
 255
 256 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 257         ({                                                              \
 258                 int ret = read_seqretry(lock, iv);                      \
 259                 local_irq_restore(flags);                               \
 260                 ret;                                                    \
 261         })
 262
 263 #endif /* __LINUX_SEQLOCK_H */