[PATCH] slow_work_execute() needs mb() beforetest_bit(SLOW_WORK_PENDING)

[Oleg Nesterov]

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@xxxxxxxxxx>

--- 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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