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1 | ==================== | |
2 | Percpu rw semaphores | |
3 | ==================== | |
4 | ||
5 | Percpu rw semaphores is a new read-write semaphore design that is | |
6 | optimized for locking for reading. | |
7 | ||
8 | The problem with traditional read-write semaphores is that when multiple | |
9 | cores take the lock for reading, the cache line containing the semaphore | |
10 | is bouncing between L1 caches of the cores, causing performance | |
11 | degradation. | |
12 | ||
13 | Locking for reading is very fast, it uses RCU and it avoids any atomic | |
14 | instruction in the lock and unlock path. On the other hand, locking for | |
15 | writing is very expensive, it calls synchronize_rcu() that can take | |
16 | hundreds of milliseconds. | |
17 | ||
18 | The lock is declared with "struct percpu_rw_semaphore" type. | |
19 | The lock is initialized percpu_init_rwsem, it returns 0 on success and | |
20 | -ENOMEM on allocation failure. | |
21 | The lock must be freed with percpu_free_rwsem to avoid memory leak. | |
22 | ||
23 | The lock is locked for read with percpu_down_read, percpu_up_read and | |
24 | for write with percpu_down_write, percpu_up_write. | |
25 | ||
26 | The idea of using RCU for optimized rw-lock was introduced by | |
27 | Eric Dumazet <eric.dumazet@gmail.com>. | |
28 | The code was written by Mikulas Patocka <mpatocka@redhat.com> |