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 = sl->sequence;
  89         smp_rmb();
  90         return ret;
  91 }
  92
  93 /* Test if reader processed invalid data.
  94  * If initial values is odd,
  95  *      then writer had already started when section was entered
  96  * If sequence value changed
  97  *      then writer changed data while in section
  98  *
  99  * Using xor saves one conditional branch.
 100  */
 101 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
 102 {
 103         smp_rmb();
 104         return (iv & 1) | (sl->sequence ^ iv);
 105 }
 106
 107
 108 /*
 109  * Version using sequence counter only.
 110  * This can be used when code has its own mutex protecting the
 111  * updating starting before the write_seqcountbeqin() and ending
 112  * after the write_seqcount_end().
 113  */
 114
 115 typedef struct seqcount {
 116         unsigned sequence;
 117 } seqcount_t;
 118
 119 #define SEQCNT_ZERO { 0 }
 120 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 121
 122 /* Start of read using pointer to a sequence counter only.  */
 123 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 124 {
 125         unsigned ret = s->sequence;
 126         smp_rmb();
 127         return ret;
 128 }
 129
 130 /* Test if reader processed invalid data.
 131  * Equivalent to: iv is odd or sequence number has changed.
 132  *                (iv & 1) || (*s != iv)
 133  * Using xor saves one conditional branch.
 134  */
 135 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
 136 {
 137         smp_rmb();
 138         return (iv & 1) | (s->sequence ^ iv);
 139 }
 140
 141
 142 /*
 143  * Sequence counter only version assumes that callers are using their
 144  * own mutexing.
 145  */
 146 static inline void write_seqcount_begin(seqcount_t *s)
 147 {
 148         s->sequence++;
 149         smp_wmb();
 150 }
 151
 152 static inline void write_seqcount_end(seqcount_t *s)
 153 {
 154         smp_wmb();
 155         s->sequence++;
 156 }
 157
 158 /*
 159  * Possible sw/hw IRQ protected versions of the interfaces.
 160  */
 161 #define write_seqlock_irqsave(lock, flags)                              \
 162         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 163 #define write_seqlock_irq(lock)                                         \
 164         do { local_irq_disable();   write_seqlock(lock); } while (0)
 165 #define write_seqlock_bh(lock)                                          \
 166         do { local_bh_disable();    write_seqlock(lock); } while (0)
 167
 168 #define write_sequnlock_irqrestore(lock, flags)                         \
 169         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 170 #define write_sequnlock_irq(lock)                                       \
 171         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 172 #define write_sequnlock_bh(lock)                                        \
 173         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 174
 175 #define read_seqbegin_irqsave(lock, flags)                              \
 176         ({ local_irq_save(flags);   read_seqbegin(lock); })
 177
 178 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 179         ({                                                              \
 180                 int ret = read_seqretry(lock, iv);                      \
 181                 local_irq_restore(flags);                               \
 182                 ret;                                                    \
 183         })
 184
 185 #endif /* __LINUX_SEQLOCK_H */