ipc/sem.c: avoid using spin_unlock_wait()
authorManfred Spraul <manfred@colorfullife.com>
Mon, 27 Feb 2017 22:28:15 +0000 (14:28 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 28 Feb 2017 02:43:46 +0000 (18:43 -0800)
commit27d7be1801a4824ecccbc735593101d72c038f13
tree3955f64e229157c2acdec238e9e5a24b7d457db8
parent08865514805d2de8e7002fa8149c5de3e391f412
ipc/sem.c: avoid using spin_unlock_wait()

a) The ACQUIRE in spin_lock() applies to the read, not to the store, at
   least for powerpc.  This forces to add a smp_mb() into the fast path.

b) The memory barrier provided by spin_unlock_wait() is right now arch
   dependent.

Therefore: Use spin_lock()/spin_unlock() instead of spin_unlock_wait().

Advantage: faster single op semop calls(), observed +8.9% on x86.  (the
other solution would be arch dependencies in ipc/sem).

Disadvantage: slower complex op semop calls, if (and only if) there are
no sleeping operations.

The next patch adds hysteresis, this further reduces the probability
that the slow path is used.

Link: http://lkml.kernel.org/r/1476851896-3590-2-git-send-email-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: <1vier1@web.de>
Cc: kernel test robot <xiaolong.ye@intel.com>
Cc: <felixh@informatik.uni-bremen.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
ipc/sem.c