[PATCH] workqueue: fix possible race condition when rescuer VS pwq-release

[Lai Jiangshan <laijs@cn.fujitsu.com>]

There is a race condition between rescuer_thread() and
pwq_unbound_release_workfn().

The works of the @pwq may be processed by some other worker,
and @pwq is scheduled to release before the rescuer starts
to process. In this case pwq_unbound_release_workfn() will
corrupt wq->maydays, and rescuer_thead will access to corrupted
data.

Using get_pwq() when send_mayday() will keep @pwq's lifetime
and avoid the race condition.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 kernel/workqueue.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 82ef9f3..7066519 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1902,6 +1902,12 @@ static void send_mayday(struct work_struct *work)
 
 	/* mayday mayday mayday */
 	if (list_empty(&pwq->mayday_node)) {
+		/*
+		 * Keep the pwq and avoid the pwq to be scheduled to release
+		 * when someone else processes all the works before the rescuer
+		 * starts to process.
+		 */
+		get_pwq(pwq);
 		list_add_tail(&pwq->mayday_node, &wq->maydays);
 		wake_up_process(wq->rescuer->task);
 	}
@@ -2418,6 +2424,7 @@ repeat:
 		/* migrate to the target cpu if possible */
 		worker_maybe_bind_and_lock(pool);
 		rescuer->pool = pool;
+		put_pwq(pwq);
 
 		/*
 		 * Slurp in all works issued via this workqueue and
-- 
1.7.7.6