destroy_workqueue() can livelock

Pointed out by Michal Schmidt <mschmidt@redhat.com>.

The bug was introduced in 2.6.22 by me.

cleanup_workqueue_thread() does flush_cpu_workqueue(cwq) in a loop until
->worklist becomes empty.  This is live-lockable, a re-niced caller can get
CPU after wake_up() and insert a new barrier before the lower-priority
cwq->thread has a chance to clear ->current_work.

Change cleanup_workqueue_thread() to do flush_cpu_workqueue(cwq) only once.
 We can rely on the fact that run_workqueue() won't return until it flushes
all works.  So it is safe to call kthread_stop() after that, the "should
stop" request won't be noticed until run_workqueue() returns.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Michal Schmidt <mschmidt@redhat.com>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/workqueue.c

@@ -752,18 +752,17 @@ static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
 	if (cwq->thread == NULL)
 		return;
 
+	flush_cpu_workqueue(cwq);
 	/*
-	 * If the caller is CPU_DEAD the single flush_cpu_workqueue()
-	 * is not enough, a concurrent flush_workqueue() can insert a
-	 * barrier after us.
+	 * If the caller is CPU_DEAD and cwq->worklist was not empty,
+	 * a concurrent flush_workqueue() can insert a barrier after us.
+	 * However, in that case run_workqueue() won't return and check
+	 * kthread_should_stop() until it flushes all work_struct's.
 	 * When ->worklist becomes empty it is safe to exit because no
 	 * more work_structs can be queued on this cwq: flush_workqueue
 	 * checks list_empty(), and a "normal" queue_work() can't use
 	 * a dead CPU.
 	 */
-	while (flush_cpu_workqueue(cwq))
-		;
-
 	kthread_stop(cwq->thread);
 	cwq->thread = NULL;
 }