linux-kernel - [PATCH 3/7] ipc/sem.c: Rely on spin_unlock_wait() = spin_lock();spin

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Date:   Thu,  1 Sep 2016 17:04:13 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>, Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, will.deacon@....com,
        1vier1@....de, Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 3/7] ipc/sem.c: Rely on spin_unlock_wait() = spin_lock();spin_unlock().

>From memory ordering point of view, spin_unlock_wait() provides
the same guarantees as spin_lock(); spin_unlock().

Therefore the smp_mb() after spin_lock() is not necessary,
spin_unlock_wait() must provide the memory ordering.

Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
Cc: Will Deacon <will.deacon@....com>
---
 ipc/sem.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 6586e0a..a5da82c 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -355,14 +355,6 @@ 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;
-- 
2.7.4