From: Andrew Morton <akpm@osdl.org>
Date: Sat, 24 Dec 2005 03:54:46 +0000 (-0800)
Subject: [PATCH] Fix memory ordering problem in wake_futex()
X-Git-Tag: v2.6.15-rc7~8
X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=8e31108b9f41069d55cb9b019ac8262c55fd2616;p=linux-2.6-omap-h63xx.git

[PATCH] Fix memory ordering problem in wake_futex()

Fix a memory ordering problem that occurs on IA64. The "store" to q->lock_ptr
in wake_futex() can become visible before wake_up_all() clears the lock in the
futex_q.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

diff --git a/kernel/futex.c b/kernel/futex.c
index 5872e3507f3..5e71a6bf6f6 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -270,7 +270,13 @@ static void wake_futex(struct futex_q *q)
 	/*
 	 * The waiting task can free the futex_q as soon as this is written,
 	 * without taking any locks.  This must come last.
+	 *
+	 * A memory barrier is required here to prevent the following store
+	 * to lock_ptr from getting ahead of the wakeup. Clearing the lock
+	 * at the end of wake_up_all() does not prevent this store from
+	 * moving.
 	 */
+	wmb();
 	q->lock_ptr = NULL;
 }