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-Percpu rw semaphores
---------------------
-
-Percpu rw semaphores is a new read-write semaphore design that is
-optimized for locking for reading.
-
-The problem with traditional read-write semaphores is that when multiple
-cores take the lock for reading, the cache line containing the semaphore
-is bouncing between L1 caches of the cores, causing performance
-degradation.
-
-Locking for reading is very fast, it uses RCU and it avoids any atomic
-instruction in the lock and unlock path. On the other hand, locking for
-writing is very expensive, it calls synchronize_rcu() that can take
-hundreds of milliseconds.
-
-The lock is declared with "struct percpu_rw_semaphore" type.
-The lock is initialized percpu_init_rwsem, it returns 0 on success and
--ENOMEM on allocation failure.
-The lock must be freed with percpu_free_rwsem to avoid memory leak.
-
-The lock is locked for read with percpu_down_read, percpu_up_read and
-for write with percpu_down_write, percpu_up_write.
-
-The idea of using RCU for optimized rw-lock was introduced by
-Eric Dumazet <eric.dumazet@gmail.com>.
-The code was written by Mikulas Patocka <mpatocka@redhat.com>