Commit 3552a07a authored Sep 30, 2015 by Davidlohr Bueso Committed by Ingo Molnar Oct 06, 2015

locking/mcs: Use acquire/release semantics



As of 654672d4 (locking/atomics: Add _{acquire|release|relaxed}()
variants of some atomic operations) and 6d79ef2d (locking, asm-generic:
Add _{relaxed|acquire|release}() variants for 'atomic_long_t'), weakly
ordered archs can benefit from more relaxed use of barriers when locking
and unlocking, instead of regular full barrier semantics. While currently
only arm64 supports such optimizations, updating corresponding locking
primitives serves for other archs to immediately benefit as well, once the
necessary machinery is implemented of course.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul E.McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1443643395-17016-5-git-send-email-dave@stgolabs.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 700318d1

kernel/locking/mcs_spinlock.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -67,7 +67,7 @@ void mcs_spin_lock(struct mcs_spinlock *lock, struct mcs_spinlock node)
		node->locked = 0;
		node->next = NULL;

		prev = xchg(lock, node);
		prev = xchg_acquire(lock, node);
		if (likely(prev == NULL)) {
		/*
		* Lock acquired, don't need to set node->locked to 1. Threads
		@@ -98,7 +98,7 @@ void mcs_spin_unlock(struct mcs_spinlock *lock, struct mcs_spinlock node)
		/*
		* Release the lock by setting it to NULL
		*/
		if (likely(cmpxchg(lock, node, NULL) == node))
		if (likely(cmpxchg_release(lock, node, NULL) == node))
		return;
		/* Wait until the next pointer is set */
		while (!(next = READ_ONCE(node->next)))