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