linux-kernel - [PATCH 2/2] ipc/sem.c: Remove duplicated memory barriers.

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Date:	Wed, 13 Jul 2016 07:06:52 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, 1vier1@....de,
	felixh@...ormatik.uni-bremen.de,
	Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 2/2] ipc/sem.c: Remove duplicated memory barriers.

With 2c610022711 (locking/qspinlock: Fix spin_unlock_wait() some more),
memory barriers were added into spin_unlock_wait().
Thus another barrier is not required.

And as explained in 055ce0fd1b8 (locking/qspinlock: Add comments),
spin_lock() provides a barrier so that reads within the critical
section cannot happen before the write for the lock is visible.
i.e. spin_lock provides an acquire barrier after the write of the lock
variable, this barrier pairs with the smp_mb() in complexmode_enter().

Please review!
For x86, the patch is safe. But I don't know enough about all archs
that support SMP.

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
---
 ipc/sem.c | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 0da63c8..d7b4212 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -291,14 +291,6 @@ static void complexmode_enter(struct sem_array *sma)
 		sem = sma->sem_base + i;
 		spin_unlock_wait(&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();
 }
 
 /*
@@ -363,12 +355,6 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 		 */
 		spin_lock(&sem->lock);
 
-		/*
-		 * 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;
-- 
2.5.5