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/config.h>
  30 #include <linux/spinlock.h>
  31 #include <linux/preempt.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 { 0, SPIN_LOCK_UNLOCKED }
  43 #define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
  44
  45
  46 /* Lock out other writers and update the count.
  47  * Acts like a normal spin_lock/unlock.
  48  * Don't need preempt_disable() because that is in the spin_lock already.
  49  */
  50 static inline void write_seqlock(seqlock_t *sl)
  51 {
  52         spin_lock(&sl->lock);
  53         ++sl->sequence;
  54         smp_wmb();
  55 }
  56
  57 static inline void write_sequnlock(seqlock_t *sl)
  58 {
  59         smp_wmb();
  60         sl->sequence++;
  61         spin_unlock(&sl->lock);
  62 }
  63
  64 static inline int write_tryseqlock(seqlock_t *sl)
  65 {
  66         int ret = spin_trylock(&sl->lock);
  67
  68         if (ret) {
  69                 ++sl->sequence;
  70                 smp_wmb();
  71         }
  72         return ret;
  73 }
  74
  75 /* Start of read calculation -- fetch last complete writer token */
  76 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  77 {
  78         unsigned ret = sl->sequence;
  79         smp_rmb();
  80         return ret;
  81 }
  82
  83 /* Test if reader processed invalid data.
  84  * If initial values is odd,
  85  *      then writer had already started when section was entered
  86  * If sequence value changed
  87  *      then writer changed data while in section
  88  *
  89  * Using xor saves one conditional branch.
  90  */
  91 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
  92 {
  93         smp_rmb();
  94         return (iv & 1) | (sl->sequence ^ iv);
  95 }
  96
  97
  98 /*
  99  * Version using sequence counter only.
 100  * This can be used when code has its own mutex protecting the
 101  * updating starting before the write_seqcountbeqin() and ending
 102  * after the write_seqcount_end().
 103  */
 104
 105 typedef struct seqcount {
 106         unsigned sequence;
 107 } seqcount_t;
 108
 109 #define SEQCNT_ZERO { 0 }
 110 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 111
 112 /* Start of read using pointer to a sequence counter only.  */
 113 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 114 {
 115         unsigned ret = s->sequence;
 116         smp_rmb();
 117         return ret;
 118 }
 119
 120 /* Test if reader processed invalid data.
 121  * Equivalent to: iv is odd or sequence number has changed.
 122  *                (iv & 1) || (*s != iv)
 123  * Using xor saves one conditional branch.
 124  */
 125 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
 126 {
 127         smp_rmb();
 128         return (iv & 1) | (s->sequence ^ iv);
 129 }
 130
 131
 132 /*
 133  * Sequence counter only version assumes that callers are using their
 134  * own mutexing.
 135  */
 136 static inline void write_seqcount_begin(seqcount_t *s)
 137 {
 138         s->sequence++;
 139         smp_wmb();
 140 }
 141
 142 static inline void write_seqcount_end(seqcount_t *s)
 143 {
 144         smp_wmb();
 145         s->sequence++;
 146 }
 147
 148 /*
 149  * Possible sw/hw IRQ protected versions of the interfaces.
 150  */
 151 #define write_seqlock_irqsave(lock, flags)                              \
 152         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 153 #define write_seqlock_irq(lock)                                         \
 154         do { local_irq_disable();   write_seqlock(lock); } while (0)
 155 #define write_seqlock_bh(lock)                                          \
 156         do { local_bh_disable();    write_seqlock(lock); } while (0)
 157
 158 #define write_sequnlock_irqrestore(lock, flags)                         \
 159         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 160 #define write_sequnlock_irq(lock)                                       \
 161         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 162 #define write_sequnlock_bh(lock)                                        \
 163         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 164
 165 #define read_seqbegin_irqsave(lock, flags)                              \
 166         ({ local_irq_save(flags);   read_seqbegin(lock); })
 167
 168 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 169         ({                                                              \
 170                 int ret = read_seqretry(lock, iv);                      \
 171                 local_irq_restore(flags);                               \
 172                 ret;                                                    \
 173         })
 174
 175 #endif /* __LINUX_SEQLOCK_H */