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1====================2Percpu rw semaphores3====================4 5Percpu rw semaphores is a new read-write semaphore design that is6optimized for locking for reading.7 8The problem with traditional read-write semaphores is that when multiple9cores take the lock for reading, the cache line containing the semaphore10is bouncing between L1 caches of the cores, causing performance11degradation.12 13Locking for reading is very fast, it uses RCU and it avoids any atomic14instruction in the lock and unlock path. On the other hand, locking for15writing is very expensive, it calls synchronize_rcu() that can take16hundreds of milliseconds.17 18The lock is declared with "struct percpu_rw_semaphore" type.19The lock is initialized percpu_init_rwsem, it returns 0 on success and20-ENOMEM on allocation failure.21The lock must be freed with percpu_free_rwsem to avoid memory leak.22 23The lock is locked for read with percpu_down_read, percpu_up_read and24for write with percpu_down_write, percpu_up_write.25 26The idea of using RCU for optimized rw-lock was introduced by27Eric Dumazet <eric.dumazet@gmail.com>.28The code was written by Mikulas Patocka <mpatocka@redhat.com>29