kernel/locking/percpu-rwsem.c

   1 #include <linux/atomic.h>
   2 #include <linux/rwsem.h>
   3 #include <linux/percpu.h>
   4 #include <linux/wait.h>
   5 #include <linux/lockdep.h>
   6 #include <linux/percpu-rwsem.h>
   7 #include <linux/rcupdate.h>
   8 #include <linux/sched.h>
   9 #include <linux/errno.h>
  10
  11 int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,
  12                         const char *name, struct lock_class_key *rwsem_key)
  13 {
  14         brw->fast_read_ctr = alloc_percpu(int);
  15         if (unlikely(!brw->fast_read_ctr))
  16                 return -ENOMEM;
  17
  18         /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
  19         __init_rwsem(&brw->rw_sem, name, rwsem_key);
  20         rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);
  21         atomic_set(&brw->slow_read_ctr, 0);
  22         init_waitqueue_head(&brw->write_waitq);
  23         return 0;
  24 }
  25 EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
  26
  27 void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
  28 {
  29         /*
  30          * XXX: temporary kludge. The error path in alloc_super()
  31          * assumes that percpu_free_rwsem() is safe after kzalloc().
  32          */
  33         if (!brw->fast_read_ctr)
  34                 return;
  35
  36         rcu_sync_dtor(&brw->rss);
  37         free_percpu(brw->fast_read_ctr);
  38         brw->fast_read_ctr = NULL; /* catch use after free bugs */
  39 }
  40
  41 /*
  42  * This is the fast-path for down_read/up_read. If it succeeds we rely
  43  * on the barriers provided by rcu_sync_enter/exit; see the comments in
  44  * percpu_down_write() and percpu_up_write().
  45  *
  46  * If this helper fails the callers rely on the normal rw_semaphore and
  47  * atomic_dec_and_test(), so in this case we have the necessary barriers.
  48  */
  49 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
  50 {
  51         bool success;
  52
  53         preempt_disable();
  54         success = rcu_sync_is_idle(&brw->rss);
  55         if (likely(success))
  56                 __this_cpu_add(*brw->fast_read_ctr, val);
  57         preempt_enable();
  58
  59         return success;
  60 }
  61
  62 /*
  63  * Like the normal down_read() this is not recursive, the writer can
  64  * come after the first percpu_down_read() and create the deadlock.
  65  *
  66  * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
  67  * percpu_up_read() does rwsem_release(). This pairs with the usage
  68  * of ->rw_sem in percpu_down/up_write().
  69  */
  70 void percpu_down_read(struct percpu_rw_semaphore *brw)
  71 {
  72         might_sleep();
  73         rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
  74
  75         if (likely(update_fast_ctr(brw, +1)))
  76                 return;
  77
  78         /* Avoid rwsem_acquire_read() and rwsem_release() */
  79         __down_read(&brw->rw_sem);
  80         atomic_inc(&brw->slow_read_ctr);
  81         __up_read(&brw->rw_sem);
  82 }
  83 EXPORT_SYMBOL_GPL(percpu_down_read);
  84
  85 int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
  86 {
  87         if (unlikely(!update_fast_ctr(brw, +1))) {
  88                 if (!__down_read_trylock(&brw->rw_sem))
  89                         return 0;
  90                 atomic_inc(&brw->slow_read_ctr);
  91                 __up_read(&brw->rw_sem);
  92         }
  93
  94         rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
  95         return 1;
  96 }
  97
  98 void percpu_up_read(struct percpu_rw_semaphore *brw)
  99 {
 100         rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
 101
 102         if (likely(update_fast_ctr(brw, -1)))
 103                 return;
 104
 105         /* false-positive is possible but harmless */
 106         if (atomic_dec_and_test(&brw->slow_read_ctr))
 107                 wake_up_all(&brw->write_waitq);
 108 }
 109 EXPORT_SYMBOL_GPL(percpu_up_read);
 110
 111 static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
 112 {
 113         unsigned int sum = 0;
 114         int cpu;
 115
 116         for_each_possible_cpu(cpu) {
 117                 sum += per_cpu(*brw->fast_read_ctr, cpu);
 118                 per_cpu(*brw->fast_read_ctr, cpu) = 0;
 119         }
 120
 121         return sum;
 122 }
 123
 124 void percpu_down_write(struct percpu_rw_semaphore *brw)
 125 {
 126         /*
 127          * Make rcu_sync_is_idle() == F and thus disable the fast-path in
 128          * percpu_down_read() and percpu_up_read(), and wait for gp pass.
 129          *
 130          * The latter synchronises us with the preceding readers which used
 131          * the fast-past, so we can not miss the result of __this_cpu_add()
 132          * or anything else inside their criticial sections.
 133          */
 134         rcu_sync_enter(&brw->rss);
 135
 136         /* exclude other writers, and block the new readers completely */
 137         down_write(&brw->rw_sem);
 138
 139         /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
 140         atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
 141
 142         /* wait for all readers to complete their percpu_up_read() */
 143         wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
 144 }
 145 EXPORT_SYMBOL_GPL(percpu_down_write);
 146
 147 void percpu_up_write(struct percpu_rw_semaphore *brw)
 148 {
 149         /* release the lock, but the readers can't use the fast-path */
 150         up_write(&brw->rw_sem);
 151         /*
 152          * Enable the fast-path in percpu_down_read() and percpu_up_read()
 153          * but only after another gp pass; this adds the necessary barrier
 154          * to ensure the reader can't miss the changes done by us.
 155          */
 156         rcu_sync_exit(&brw->rss);
 157 }
 158 EXPORT_SYMBOL_GPL(percpu_up_write);