linux-kernel - [PATCH 2/4] workqueue: fix race condition in flush

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Message-ID: <4C2DA4A5.10107@gmail.com>
Date:	Fri, 02 Jul 2010 10:34:45 +0200
From:	Tejun Heo <htejun@...il.com>
To:	torvalds@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, rusty@...tcorp.com.au,
	cl@...ux-foundation.org, dhowells@...hat.com,
	arjan@...ux.intel.com, oleg@...hat.com, axboe@...nel.dk,
	fweisbec@...il.com, dwalker@...eaurora.org,
	stefanr@...6.in-berlin.de, florian@...kler.org,
	andi@...stfloor.org, mst@...hat.com, randy.dunlap@...cle.com
Subject: [PATCH 2/4] workqueue: fix race condition in flush_workqueue()

When one flusher is cascading to the next flusher, it first sets
wq->first_flusher to the next one and sets up the next flush cycle.
If there's nothing to do for the next cycle, it clears
wq->flush_flusher and proceeds to the one after that.

If the woken up flusher checks wq->first_flusher before it gets
cleared, it will incorrectly assume the role of the first flusher,
which triggers BUG_ON() sanity check.

Fix it by checking wq->first_flusher again after grabbing the mutex.

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 kernel/workqueue.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5587338..b59c946 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2138,6 +2138,10 @@ void flush_workqueue(struct workqueue_struct *wq)

 	mutex_lock(&wq->flush_mutex);

+	/* we might have raced, check again with mutex held */
+	if (wq->first_flusher != &this_flusher)
+		goto out_unlock;
+
 	wq->first_flusher = NULL;

 	BUG_ON(!list_empty(&this_flusher.list));
-- 
1.6.4.2

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