linux/Documentation/percpu-rw-semaphore.txt
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   1Percpu rw semaphores
   2--------------------
   3
   4Percpu rw semaphores is a new read-write semaphore design that is
   5optimized for locking for reading.
   6
   7The problem with traditional read-write semaphores is that when multiple
   8cores take the lock for reading, the cache line containing the semaphore
   9is bouncing between L1 caches of the cores, causing performance
  10degradation.
  11
  12Locking for reading is very fast, it uses RCU and it avoids any atomic
  13instruction in the lock and unlock path. On the other hand, locking for
  14writing is very expensive, it calls synchronize_rcu() that can take
  15hundreds of milliseconds.
  16
  17The lock is declared with "struct percpu_rw_semaphore" type.
  18The lock is initialized percpu_init_rwsem, it returns 0 on success and
  19-ENOMEM on allocation failure.
  20The lock must be freed with percpu_free_rwsem to avoid memory leak.
  21
  22The lock is locked for read with percpu_down_read, percpu_up_read and
  23for write with percpu_down_write, percpu_up_write.
  24
  25The idea of using RCU for optimized rw-lock was introduced by
  26Eric Dumazet <eric.dumazet@gmail.com>.
  27The code was written by Mikulas Patocka <mpatocka@redhat.com>
  28
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