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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>
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