linux-kernel - Problem with freezable workqueues

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Message-Id: <200702272251.28844.rjw@sisk.pl>
Date:	Tue, 27 Feb 2007 22:51:27 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Johannes Berg <johannes@...solutions.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>
Subject: Problem with freezable workqueues

Hi,

We have a problem with freezable workqueues in 2.6.21-rc1 and in -mm
(there are only two of them, in XFS, but still).  Namely, their worker threads
deadlock with workqueue_cpu_callback() that gets called during the CPU hotplug,
becuase workqueue_cpu_callback() tries to stop these threads while they are
frozen (disable_nonboot_cpus() happens after we've frozen tasks).

For 2.6.21-rc1 I've invented the appended workaround (works for me, waiting for
Johannes to confirm it works for him too), but I think we need something better
for -mm and future kernels.

Greetings,
Rafael


 kernel/workqueue.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

Index: linux-2.6.21-rc1/kernel/workqueue.c
===================================================================
--- linux-2.6.21-rc1.orig/kernel/workqueue.c	2007-02-24 10:17:57.000000000 +0100
+++ linux-2.6.21-rc1/kernel/workqueue.c	2007-02-24 20:00:22.000000000 +0100
@@ -376,8 +376,19 @@ static int worker_thread(void *__cwq)
 
 	set_current_state(TASK_INTERRUPTIBLE);
 	while (!kthread_should_stop()) {
-		if (cwq->freezeable)
-			try_to_freeze();
+		if (try_to_freeze()) {
+			/* We've just left the refrigerator.  If our CPU is
+			 * a nonboot one, we might have been replaced.
+			 * The lock is taken to prevent the race with
+			 * cleanup_workqueue_thread() from happening
+			 */
+			spin_lock_irq(&cwq->lock);
+			if (cwq->thread != current) {
+				spin_unlock_irq(&cwq->lock);
+				break;
+			}
+			spin_unlock_irq(&cwq->lock);
+		}
 
 		add_wait_queue(&cwq->more_work, &wait);
 		if (list_empty(&cwq->worklist))
@@ -539,6 +550,9 @@ static void cleanup_workqueue_thread(str
 	cwq = per_cpu_ptr(wq->cpu_wq, cpu);
 	spin_lock_irqsave(&cwq->lock, flags);
 	p = cwq->thread;
+	if (p && frozen(p))
+		p = NULL;
+
 	cwq->thread = NULL;
 	spin_unlock_irqrestore(&cwq->lock, flags);
 	if (p)
-
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