| 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> |
