kitten/include/lwk/seqlock.h

   1 #ifndef _LWK_SEQLOCK_H
   2 #define _LWK_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 <lwk/spinlock.h>
  30
  31 typedef struct {
  32         unsigned sequence;
  33         spinlock_t lock;
  34 } seqlock_t;
  35
  36 /*
  37  * These macros triggered gcc-3.x compile-time problems.  We think these are
  38  * OK now.  Be cautious.
  39  */
  40 #define SEQLOCK_UNLOCKED { 0, SPIN_LOCK_UNLOCKED }
  41 #define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
  42
  43
  44 /* Lock out other writers and update the count.
  45  * Acts like a normal spin_lock/unlock.
  46  */
  47 static inline void write_seqlock(seqlock_t *sl)
  48 {
  49         spin_lock(&sl->lock);
  50         ++sl->sequence;
  51         smp_wmb();
  52 }
  53
  54 static inline void write_sequnlock(seqlock_t *sl)
  55 {
  56         smp_wmb();
  57         sl->sequence++;
  58         spin_unlock(&sl->lock);
  59 }
  60
  61 static inline int write_tryseqlock(seqlock_t *sl)
  62 {
  63         int ret = spin_trylock(&sl->lock);
  64
  65         if (ret) {
  66                 ++sl->sequence;
  67                 smp_wmb();
  68         }
  69         return ret;
  70 }
  71
  72 /* Start of read calculation -- fetch last complete writer token */
  73 static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
  74 {
  75         unsigned ret = sl->sequence;
  76         smp_rmb();
  77         return ret;
  78 }
  79
  80 /* Test if reader processed invalid data.
  81  * If initial values is odd,
  82  *      then writer had already started when section was entered
  83  * If sequence value changed
  84  *      then writer changed data while in section
  85  *
  86  * Using xor saves one conditional branch.
  87  */
  88 static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
  89 {
  90         smp_rmb();
  91         return (iv & 1) | (sl->sequence ^ iv);
  92 }
  93
  94
  95 /*
  96  * Version using sequence counter only.
  97  * This can be used when code has its own mutex protecting the
  98  * updating starting before the write_seqcountbeqin() and ending
  99  * after the write_seqcount_end().
 100  */
 101
 102 typedef struct seqcount {
 103         unsigned sequence;
 104 } seqcount_t;
 105
 106 #define SEQCNT_ZERO { 0 }
 107 #define seqcount_init(x)        do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
 108
 109 /* Start of read using pointer to a sequence counter only.  */
 110 static inline unsigned read_seqcount_begin(const seqcount_t *s)
 111 {
 112         unsigned ret = s->sequence;
 113         smp_rmb();
 114         return ret;
 115 }
 116
 117 /* Test if reader processed invalid data.
 118  * Equivalent to: iv is odd or sequence number has changed.
 119  *                (iv & 1) || (*s != iv)
 120  * Using xor saves one conditional branch.
 121  */
 122 static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
 123 {
 124         smp_rmb();
 125         return (iv & 1) | (s->sequence ^ iv);
 126 }
 127
 128
 129 /*
 130  * Sequence counter only version assumes that callers are using their
 131  * own mutexing.
 132  */
 133 static inline void write_seqcount_begin(seqcount_t *s)
 134 {
 135         s->sequence++;
 136         smp_wmb();
 137 }
 138
 139 static inline void write_seqcount_end(seqcount_t *s)
 140 {
 141         smp_wmb();
 142         s->sequence++;
 143 }
 144
 145 /*
 146  * Possible sw/hw IRQ protected versions of the interfaces.
 147  */
 148 #define write_seqlock_irqsave(lock, flags)                              \
 149         do { local_irq_save(flags); write_seqlock(lock); } while (0)
 150 #define write_seqlock_irq(lock)                                         \
 151         do { local_irq_disable();   write_seqlock(lock); } while (0)
 152 #define write_seqlock_bh(lock)                                          \
 153         do { local_bh_disable();    write_seqlock(lock); } while (0)
 154
 155 #define write_sequnlock_irqrestore(lock, flags)                         \
 156         do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
 157 #define write_sequnlock_irq(lock)                                       \
 158         do { write_sequnlock(lock); local_irq_enable(); } while(0)
 159 #define write_sequnlock_bh(lock)                                        \
 160         do { write_sequnlock(lock); local_bh_enable(); } while(0)
 161
 162 #define read_seqbegin_irqsave(lock, flags)                              \
 163         ({ local_irq_save(flags);   read_seqbegin(lock); })
 164
 165 #define read_seqretry_irqrestore(lock, iv, flags)                       \
 166         ({                                                              \
 167                 int ret = read_seqretry(lock, iv);                      \
 168                 local_irq_restore(flags);                               \
 169                 ret;                                                    \
 170         })
 171
 172 #endif /* _LWK_SEQLOCK_H */