linux-kernel - [PATCH] ipc/sem.c: [RFC] memory barrier in sem

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Message-Id: <1407872586-2738-1-git-send-email-manfred@colorfullife.com>
Date:	Tue, 12 Aug 2014 21:43:06 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Rik van Riel <riel@...hat.com>, 1vier1@....de,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH] ipc/sem.c: [RFC] memory barrier in sem_lock()

sem_lock right now contains an smp_mb().
I think smp_rmb() would be sufficient - and performance of semop() with rmb()
is up to 10% faster. It would be a pairing of rmb() with spin_unlock().

The race we must protect against is:

sem->lock is free
sma->complex_count = 0
sma->sem_perm.lock held by thread B

thread A:

A: spin_lock(&sem->lock)

			B: sma->complex_count++; (now 1)
			B: spin_unlock(&sma->sem_perm.lock);

A: spin_is_locked(&sma->sem_perm.lock);
A: XXXXX which memory barrier is necessary?
A: if (sma->complex_count == 0)

Thread A must read the increased complex_count value, i.e. the read must
not be reordered with the read of sem_perm.lock done by spin_is_locked().

But that's it, there are no writes that must be ordered.

---
 ipc/sem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 454f6c6..a5c8a77 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -327,7 +327,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 		/* Then check that the global lock is free */
 		if (!spin_is_locked(&sma->sem_perm.lock)) {
 			/* spin_is_locked() is not a memory barrier */
-			smp_mb();
+			smp_rmb();
 
 			/* Now repeat the test of complex_count:
 			 * It can't change anymore until we drop sem->lock.
-- 
1.9.3

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