Message	SHA1
percpu-rw-semaphores: use rcu_read_lock_sched ... Use rcu_read_lock_sched / rcu_read_unlock_sched / synchronize_sched instead of rcu_read_lock / rcu_read_unlock / synchronize_rcu. This is an optimization. The RCU-protected region is very small, so there will be no latency problems if we disable preempt in this region. So we use rcu_read_lock_sched / rcu_read_unlock_sched that translates to preempt_disable / preempt_disable. It is smaller (and supposedly faster) than preemptible rcu_read_lock / rcu_read_unlock. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Mikulas Patocka   authored and   Linus Torvalds   commited 2012-10-28 10:59:36 -07:00	1bf11c5353
percpu-rw-semaphores: use light/heavy barriers ... This patch introduces new barrier pair light_mb() and heavy_mb() for percpu rw semaphores. This patch fixes a bug in percpu-rw-semaphores where a barrier was missing in percpu_up_write. This patch improves performance on the read path of percpu-rw-semaphores: on non-x86 cpus, there was a smp_mb() in percpu_up_read. This patch changes it to a compiler barrier and removes the "#if defined(X86) ..." condition. From: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Mikulas Patocka   authored and   Linus Torvalds   commited 2012-10-28 10:59:36 -07:00	5c1eabe685
blockdev: turn a rw semaphore into a percpu rw semaphore ... This avoids cache line bouncing when many processes lock the semaphore for read. New percpu lock implementation The lock consists of an array of percpu unsigned integers, a boolean variable and a mutex. When we take the lock for read, we enter rcu read section, check for a "locked" variable. If it is false, we increase a percpu counter on the current cpu and exit the rcu section. If "locked" is true, we exit the rcu section, take the mutex and drop it (this waits until a writer finished) and retry. Unlocking for read just decreases percpu variable. Note that we can unlock on a difference cpu than where we locked, in this case the counter underflows. The sum of all percpu counters represents the number of processes that hold the lock for read. When we need to lock for write, we take the mutex, set "locked" variable to true and synchronize rcu. Since RCU has been synchronized, no processes can create new read locks. We wait until the sum of percpu counters is zero - when it is, there are no readers in the critical section. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> Mikulas Patocka   authored and   Jens Axboe   commited 2012-09-26 07:46:43 +02:00	62ac665ff9