linux-kernel - [PATCH] destroy_workqueue() can livelock

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 13 Jul 2007 17:16:55 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Michal Schmidt <mschmidt@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>, stable@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] destroy_workqueue() can livelock

Pointed out by Michal Schmidt <mschmidt@...hat.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@...sign.ru>

--- t/kernel/workqueue.c~LIVELOCK	2007-06-13 18:26:56.000000000 +0400
+++ t/kernel/workqueue.c	2007-07-13 16:46:27.000000000 +0400
@@ -739,18 +739,17 @@ static void cleanup_workqueue_thread(str
 	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;
 }

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/