|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
|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 <email@example.com>.
|The code was written by Mikulas Patocka <firstname.lastname@example.org>