lib/lockref.c

   1 #include <linux/export.h>
   2 #include <linux/lockref.h>
   3
   4 #if USE_CMPXCHG_LOCKREF
   5
   6 /*
   7  * Allow weakly-ordered memory architectures to provide barrier-less
   8  * cmpxchg semantics for lockref updates.
   9  */
  10 #ifndef cmpxchg64_relaxed
  11 # define cmpxchg64_relaxed cmpxchg64
  12 #endif
  13
  14 /*
  15  * Allow architectures to override the default cpu_relax() within CMPXCHG_LOOP.
  16  * This is useful for architectures with an expensive cpu_relax().
  17  */
  18 #ifndef arch_mutex_cpu_relax
  19 # define arch_mutex_cpu_relax() cpu_relax()
  20 #endif
  21
  22 /*
  23  * Note that the "cmpxchg()" reloads the "old" value for the
  24  * failure case.
  25  */
  26 #define CMPXCHG_LOOP(CODE, SUCCESS) do {                                        \
  27         struct lockref old;                                                     \
  28         BUILD_BUG_ON(sizeof(old) != 8);                                         \
  29         old.lock_count = ACCESS_ONCE(lockref->lock_count);                      \
  30         while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {     \
  31                 struct lockref new = old, prev = old;                           \
  32                 CODE                                                            \
  33                 old.lock_count = cmpxchg64_relaxed(&lockref->lock_count,        \
  34                                                    old.lock_count,              \
  35                                                    new.lock_count);             \
  36                 if (likely(old.lock_count == prev.lock_count)) {                \
  37                         SUCCESS;                                                \
  38                 }                                                               \
  39                 arch_mutex_cpu_relax();                                         \
  40         }                                                                       \
  41 } while (0)
  42
  43 #else
  44
  45 #define CMPXCHG_LOOP(CODE, SUCCESS) do { } while (0)
  46
  47 #endif
  48
  49 /**
  50  * lockref_get - Increments reference count unconditionally
  51  * @lockref: pointer to lockref structure
  52  *
  53  * This operation is only valid if you already hold a reference
  54  * to the object, so you know the count cannot be zero.
  55  */
  56 void lockref_get(struct lockref *lockref)
  57 {
  58         CMPXCHG_LOOP(
  59                 new.count++;
  60         ,
  61                 return;
  62         );
  63
  64         spin_lock(&lockref->lock);
  65         lockref->count++;
  66         spin_unlock(&lockref->lock);
  67 }
  68 EXPORT_SYMBOL(lockref_get);
  69
  70 /**
  71  * lockref_get_not_zero - Increments count unless the count is 0
  72  * @lockref: pointer to lockref structure
  73  * Return: 1 if count updated successfully or 0 if count was zero
  74  */
  75 int lockref_get_not_zero(struct lockref *lockref)
  76 {
  77         int retval;
  78
  79         CMPXCHG_LOOP(
  80                 new.count++;
  81                 if (!old.count)
  82                         return 0;
  83         ,
  84                 return 1;
  85         );
  86
  87         spin_lock(&lockref->lock);
  88         retval = 0;
  89         if (lockref->count) {
  90                 lockref->count++;
  91                 retval = 1;
  92         }
  93         spin_unlock(&lockref->lock);
  94         return retval;
  95 }
  96 EXPORT_SYMBOL(lockref_get_not_zero);
  97
  98 /**
  99  * lockref_get_or_lock - Increments count unless the count is 0
 100  * @lockref: pointer to lockref structure
 101  * Return: 1 if count updated successfully or 0 if count was zero
 102  * and we got the lock instead.
 103  */
 104 int lockref_get_or_lock(struct lockref *lockref)
 105 {
 106         CMPXCHG_LOOP(
 107                 new.count++;
 108                 if (!old.count)
 109                         break;
 110         ,
 111                 return 1;
 112         );
 113
 114         spin_lock(&lockref->lock);
 115         if (!lockref->count)
 116                 return 0;
 117         lockref->count++;
 118         spin_unlock(&lockref->lock);
 119         return 1;
 120 }
 121 EXPORT_SYMBOL(lockref_get_or_lock);
 122
 123 /**
 124  * lockref_put_or_lock - decrements count unless count <= 1 before decrement
 125  * @lockref: pointer to lockref structure
 126  * Return: 1 if count updated successfully or 0 if count <= 1 and lock taken
 127  */
 128 int lockref_put_or_lock(struct lockref *lockref)
 129 {
 130         CMPXCHG_LOOP(
 131                 new.count--;
 132                 if (old.count <= 1)
 133                         break;
 134         ,
 135                 return 1;
 136         );
 137
 138         spin_lock(&lockref->lock);
 139         if (lockref->count <= 1)
 140                 return 0;
 141         lockref->count--;
 142         spin_unlock(&lockref->lock);
 143         return 1;
 144 }
 145 EXPORT_SYMBOL(lockref_put_or_lock);
 146
 147 /**
 148  * lockref_mark_dead - mark lockref dead
 149  * @lockref: pointer to lockref structure
 150  */
 151 void lockref_mark_dead(struct lockref *lockref)
 152 {
 153         assert_spin_locked(&lockref->lock);
 154         lockref->count = -128;
 155 }
 156 EXPORT_SYMBOL(lockref_mark_dead);
 157
 158 /**
 159  * lockref_get_not_dead - Increments count unless the ref is dead
 160  * @lockref: pointer to lockref structure
 161  * Return: 1 if count updated successfully or 0 if lockref was dead
 162  */
 163 int lockref_get_not_dead(struct lockref *lockref)
 164 {
 165         int retval;
 166
 167         CMPXCHG_LOOP(
 168                 new.count++;
 169                 if ((int)old.count < 0)
 170                         return 0;
 171         ,
 172                 return 1;
 173         );
 174
 175         spin_lock(&lockref->lock);
 176         retval = 0;
 177         if ((int) lockref->count >= 0) {
 178                 lockref->count++;
 179                 retval = 1;
 180         }
 181         spin_unlock(&lockref->lock);
 182         return retval;
 183 }
 184 EXPORT_SYMBOL(lockref_get_not_dead);