linux-kernel - [PATCH] slow_work_execute() needs mb() before test_bit(SLOW_WORK

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Message-ID: <20090413201306.GA16653@redhat.com>
Date:	Mon, 13 Apr 2009 22:13:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Miller <davem@...emloft.net>
Cc:	Serge Hallyn <serue@...ibm.com>, Steve Dickson <steved@...hat.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Daire Byrne <Daire.Byrne@...mestore.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] slow_work_execute() needs mb() before
	test_bit(SLOW_WORK_PENDING)

slow_work_execute:

	clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);

	if (test_bit(SLOW_WORK_PENDING, &work->flags) {

clear_bit_unlock() implies release semantics, iow we have a one-way barrier
before clear_bit(). But we need the mb() semantics after clear_bit(), before
we test SLOW_WORK_PENDING. Otherwise we can miss SLOW_WORK_ENQ_DEFERRED if
we race slow_work_enqueue().

Signed-off-by: Oleg Nesterov <oleg@...hat.com>

--- 6.30/kernel/slow-work.c~2_BITS_MB	2009-04-13 19:40:20.000000000 +0200
+++ 6.30/kernel/slow-work.c	2009-04-13 21:19:33.000000000 +0200
@@ -198,7 +198,8 @@ static bool slow_work_execute(void)
 
 	if (very_slow)
 		atomic_dec(&vslow_work_executing_count);
-	clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);
+	clear_bit(SLOW_WORK_EXECUTING, &work->flags);
+	smp_mb__after_clear_bit();
 
 	/* if someone tried to enqueue the item whilst we were executing it,
 	 * then it'll be left unenqueued to avoid multiple threads trying to

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