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 EXPORT_SYMBOL_GPL(percpu_free_rwsem);
  41
  42 /*
  43  * This is the fast-path for down_read/up_read. If it succeeds we rely
  44  * on the barriers provided by rcu_sync_enter/exit; see the comments in
  45  * percpu_down_write() and percpu_up_write().
  46  *
  47  * If this helper fails the callers rely on the normal rw_semaphore and
  48  * atomic_dec_and_test(), so in this case we have the necessary barriers.
  49  */
  50 static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
  51 {
  52         bool success;
  53
  54         preempt_disable();
  55         success = rcu_sync_is_idle(&brw->rss);
  56         if (likely(success))
  57                 __this_cpu_add(*brw->fast_read_ctr, val);
  58         preempt_enable();
  59
  60         return success;
  61 }
  62
  63 /*
  64  * Like the normal down_read() this is not recursive, the writer can
  65  * come after the first percpu_down_read() and create the deadlock.
  66  *
  67  * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep,
  68  * percpu_up_read() does rwsem_release(). This pairs with the usage
  69  * of ->rw_sem in percpu_down/up_write().
  70  */
  71 void percpu_down_read(struct percpu_rw_semaphore *brw)
  72 {
  73         might_sleep();
  74         rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
  75
  76         if (likely(update_fast_ctr(brw, +1)))
  77                 return;
  78
  79         /* Avoid rwsem_acquire_read() and rwsem_release() */
  80         __down_read(&brw->rw_sem);
  81         atomic_inc(&brw->slow_read_ctr);
  82         __up_read(&brw->rw_sem);
  83 }
  84 EXPORT_SYMBOL_GPL(percpu_down_read);
  85
  86 int percpu_down_read_trylock(struct percpu_rw_semaphore *brw)
  87 {
  88         if (unlikely(!update_fast_ctr(brw, +1))) {
  89                 if (!__down_read_trylock(&brw->rw_sem))
  90                         return 0;
  91                 atomic_inc(&brw->slow_read_ctr);
  92                 __up_read(&brw->rw_sem);
  93         }
  94
  95         rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);
  96         return 1;
  97 }
  98
  99 void percpu_up_read(struct percpu_rw_semaphore *brw)
 100 {
 101         rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_);
 102
 103         if (likely(update_fast_ctr(brw, -1)))
 104                 return;
 105
 106         /* false-positive is possible but harmless */
 107         if (atomic_dec_and_test(&brw->slow_read_ctr))
 108                 wake_up_all(&brw->write_waitq);
 109 }
 110 EXPORT_SYMBOL_GPL(percpu_up_read);
 111
 112 static int clear_fast_ctr(struct percpu_rw_semaphore *brw)
 113 {
 114         unsigned int sum = 0;
 115         int cpu;
 116
 117         for_each_possible_cpu(cpu) {
 118                 sum += per_cpu(*brw->fast_read_ctr, cpu);
 119                 per_cpu(*brw->fast_read_ctr, cpu) = 0;
 120         }
 121
 122         return sum;
 123 }
 124
 125 void percpu_down_write(struct percpu_rw_semaphore *brw)
 126 {
 127         /*
 128          * Make rcu_sync_is_idle() == F and thus disable the fast-path in
 129          * percpu_down_read() and percpu_up_read(), and wait for gp pass.
 130          *
 131          * The latter synchronises us with the preceding readers which used
 132          * the fast-past, so we can not miss the result of __this_cpu_add()
 133          * or anything else inside their criticial sections.
 134          */
 135         rcu_sync_enter(&brw->rss);
 136
 137         /* exclude other writers, and block the new readers completely */
 138         down_write(&brw->rw_sem);
 139
 140         /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */
 141         atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr);
 142
 143         /* wait for all readers to complete their percpu_up_read() */
 144         wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));
 145 }
 146 EXPORT_SYMBOL_GPL(percpu_down_write);
 147
 148 void percpu_up_write(struct percpu_rw_semaphore *brw)
 149 {
 150         /* release the lock, but the readers can't use the fast-path */
 151         up_write(&brw->rw_sem);
 152         /*
 153          * Enable the fast-path in percpu_down_read() and percpu_up_read()
 154          * but only after another gp pass; this adds the necessary barrier
 155          * to ensure the reader can't miss the changes done by us.
 156          */
 157         rcu_sync_exit(&brw->rss);
 158 }
 159 EXPORT_SYMBOL_GPL(percpu_up_write);