Commit 27d7be18 authored Feb 27, 2017 by Manfred Spraul Committed by Linus Torvalds Feb 27, 2017

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>

parent 08865514

ipc/sem.c

+3 −22

Original line number	Diff line number	Diff line
		@@ -278,24 +278,13 @@ static void complexmode_enter(struct sem_array *sma)
		return;
		}

		/* We need a full barrier after seting complex_mode:
		* The write to complex_mode must be visible
		* before we read the first sem->lock spinlock state.
		*/
		smp_store_mb(sma->complex_mode, true);
		sma->complex_mode = true;

		for (i = 0; i < sma->sem_nsems; i++) {
		sem = sma->sem_base + i;
		spin_unlock_wait(&sem->lock);
		spin_lock(&sem->lock);
		spin_unlock(&sem->lock);
		}
		/*
		* spin_unlock_wait() is not a memory barriers, it is only a
		* control barrier. The code must pair with spin_unlock(&sem->lock),
		* thus just the control barrier is insufficient.
		*
		* smp_rmb() is sufficient, as writes cannot pass the control barrier.
		*/
		smp_rmb();
		}

		/*
		@@ -361,14 +350,6 @@ static inline int sem_lock(struct sem_array sma, struct sembuf sops,
		*/
		spin_lock(&sem->lock);

		/*
		* See 51d7d5205d33
		* ("powerpc: Add smp_mb() to arch_spin_is_locked()"):
		* A full barrier is required: the write of sem->lock
		* must be visible before the read is executed
		*/
		smp_mb();

		if (!smp_load_acquire(&sma->complex_mode)) {
		/* fast path successful! */
		return sops->sem_num;

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