include/linux/percpu-rwsem.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_PERCPU_RWSEM_H
   3 #define _LINUX_PERCPU_RWSEM_H
   4
   5 #include <linux/atomic.h>
   6 #include <linux/percpu.h>
   7 #include <linux/rcuwait.h>
   8 #include <linux/wait.h>
   9 #include <linux/rcu_sync.h>
  10 #include <linux/lockdep.h>
  11
  12 struct percpu_rw_semaphore {
  13         struct rcu_sync         rss;
  14         unsigned int __percpu   *read_count;
  15         struct rcuwait          writer;
  16         wait_queue_head_t       waiters;
  17         atomic_t                block;
  18 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  19         struct lockdep_map      dep_map;
  20 #endif
  21 };
  22
  23 #ifdef CONFIG_DEBUG_LOCK_ALLOC
  24 #define __PERCPU_RWSEM_DEP_MAP_INIT(lockname)   .dep_map = { .name = #lockname },
  25 #else
  26 #define __PERCPU_RWSEM_DEP_MAP_INIT(lockname)
  27 #endif
  28
  29 #define __DEFINE_PERCPU_RWSEM(name, is_static)                          \
  30 static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_rc_##name);          \
  31 is_static struct percpu_rw_semaphore name = {                           \
  32         .rss = __RCU_SYNC_INITIALIZER(name.rss),                        \
  33         .read_count = &__percpu_rwsem_rc_##name,                        \
  34         .writer = __RCUWAIT_INITIALIZER(name.writer),                   \
  35         .waiters = __WAIT_QUEUE_HEAD_INITIALIZER(name.waiters),         \
  36         .block = ATOMIC_INIT(0),                                        \
  37         __PERCPU_RWSEM_DEP_MAP_INIT(name)                               \
  38 }
  39
  40 #define DEFINE_PERCPU_RWSEM(name)               \
  41         __DEFINE_PERCPU_RWSEM(name, /* not static */)
  42 #define DEFINE_STATIC_PERCPU_RWSEM(name)        \
  43         __DEFINE_PERCPU_RWSEM(name, static)
  44
  45 extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
  46
  47 static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
  48 {
  49         might_sleep();
  50
  51         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
  52
  53         preempt_disable();
  54         /*
  55          * We are in an RCU-sched read-side critical section, so the writer
  56          * cannot both change sem->state from readers_fast and start checking
  57          * counters while we are here. So if we see !sem->state, we know that
  58          * the writer won't be checking until we're past the preempt_enable()
  59          * and that once the synchronize_rcu() is done, the writer will see
  60          * anything we did within this RCU-sched read-size critical section.
  61          */
  62         if (likely(rcu_sync_is_idle(&sem->rss)))
  63                 this_cpu_inc(*sem->read_count);
  64         else
  65                 __percpu_down_read(sem, false); /* Unconditional memory barrier */
  66         /*
  67          * The preempt_enable() prevents the compiler from
  68          * bleeding the critical section out.
  69          */
  70         preempt_enable();
  71 }
  72
  73 static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
  74 {
  75         bool ret = true;
  76
  77         preempt_disable();
  78         /*
  79          * Same as in percpu_down_read().
  80          */
  81         if (likely(rcu_sync_is_idle(&sem->rss)))
  82                 this_cpu_inc(*sem->read_count);
  83         else
  84                 ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
  85         preempt_enable();
  86         /*
  87          * The barrier() from preempt_enable() prevents the compiler from
  88          * bleeding the critical section out.
  89          */
  90
  91         if (ret)
  92                 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
  93
  94         return ret;
  95 }
  96
  97 static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
  98 {
  99         rwsem_release(&sem->dep_map, _RET_IP_);
 100
 101         preempt_disable();
 102         /*
 103          * Same as in percpu_down_read().
 104          */
 105         if (likely(rcu_sync_is_idle(&sem->rss))) {
 106                 this_cpu_dec(*sem->read_count);
 107         } else {
 108                 /*
 109                  * slowpath; reader will only ever wake a single blocked
 110                  * writer.
 111                  */
 112                 smp_mb(); /* B matches C */
 113                 /*
 114                  * In other words, if they see our decrement (presumably to
 115                  * aggregate zero, as that is the only time it matters) they
 116                  * will also see our critical section.
 117                  */
 118                 this_cpu_dec(*sem->read_count);
 119                 rcuwait_wake_up(&sem->writer);
 120         }
 121         preempt_enable();
 122 }
 123
 124 extern void percpu_down_write(struct percpu_rw_semaphore *);
 125 extern void percpu_up_write(struct percpu_rw_semaphore *);
 126
 127 extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
 128                                 const char *, struct lock_class_key *);
 129
 130 extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
 131
 132 #define percpu_init_rwsem(sem)                                  \
 133 ({                                                              \
 134         static struct lock_class_key rwsem_key;                 \
 135         __percpu_init_rwsem(sem, #sem, &rwsem_key);             \
 136 })
 137
 138 #define percpu_rwsem_is_held(sem)       lockdep_is_held(sem)
 139 #define percpu_rwsem_assert_held(sem)   lockdep_assert_held(sem)
 140
 141 static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
 142                                         bool read, unsigned long ip)
 143 {
 144         lock_release(&sem->dep_map, ip);
 145 }
 146
 147 static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
 148                                         bool read, unsigned long ip)
 149 {
 150         lock_acquire(&sem->dep_map, 0, 1, read, 1, NULL, ip);
 151 }
 152
 153 #endif