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 consitent 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 { *(x) = (seqlock_t) __SEQLOCK_UNLOCKED(x); } 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 = sl->sequence;
  86         smp_rmb();
  87         return ret;
  88 }
  89
  90 /* Test if reader processed invalid data.
  91  * If initial values is odd,
  92  *      then writer had already started when section was entered
  93  * If sequence value changed
  94  *      then writer changed data while in section
  95  *
  96  * Using xor saves one conditional branch.
  97  */
  98 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
  99 {
 100         smp_rmb();
 101         return (iv & 1) | (sl->sequence ^ iv);
 102 }
 103
 104
 105 /*
 106  * Version using sequence counter only.
 107  * This can be used when code has its own mutex protecting the
 108  * updating starting before the write_seqcountbeqin() and ending
 109  * after the write_seqcount_end().
 110  */
 111
 112 typedef struct seqcount {
 113         unsigned sequence;
 114 } seqcount_t;
 115
 116 #define SEQCNT_ZERO { 0 }
 117 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 118
 119 /* Start of read using pointer to a sequence counter only.  */
 120 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 121 {
 122         unsigned ret = s->sequence;
 123         smp_rmb();
 124         return ret;
 125 }
 126
 127 /* Test if reader processed invalid data.
 128  * Equivalent to: iv is odd or sequence number has changed.
 129  *                (iv & 1) || (*s != iv)
 130  * Using xor saves one conditional branch.
 131  */
 132 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
 133 {
 134         smp_rmb();
 135         return (iv & 1) | (s->sequence ^ iv);
 136 }
 137
 138
 139 /*
 140  * Sequence counter only version assumes that callers are using their
 141  * own mutexing.
 142  */
 143 static inline void write_seqcount_begin(seqcount_t *s)
 144 {
 145         s->sequence++;
 146         smp_wmb();
 147 }
 148
 149 static inline void write_seqcount_end(seqcount_t *s)
 150 {
 151         smp_wmb();
 152         s->sequence++;
 153 }
 154
 155 /*
 156  * Possible sw/hw IRQ protected versions of the interfaces.
 157  */
 158 #define write_seqlock_irqsave(lock, flags)                              \
 159         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 160 #define write_seqlock_irq(lock)                                         \
 161         do { local_irq_disable();   write_seqlock(lock); } while (0)
 162 #define write_seqlock_bh(lock)                                          \
 163         do { local_bh_disable();    write_seqlock(lock); } while (0)
 164
 165 #define write_sequnlock_irqrestore(lock, flags)                         \
 166         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 167 #define write_sequnlock_irq(lock)                                       \
 168         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 169 #define write_sequnlock_bh(lock)                                        \
 170         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 171
 172 #define read_seqbegin_irqsave(lock, flags)                              \
 173         ({ local_irq_save(flags);   read_seqbegin(lock); })
 174
 175 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 176         ({                                                              \
 177                 int ret = read_seqretry(lock, iv);                      \
 178                 local_irq_restore(flags);                               \
 179                 ret;                                                    \
 180         })
 181
 182 #endif /* __LINUX_SEQLOCK_H */