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