linux-kernel - Re: [PATCH] workqueue: fix cwq->nr

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Date:	Wed, 25 Aug 2010 11:12:02 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Johannes Berg <johannes@...solutions.net>
CC:	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] workqueue: fix cwq->nr_active underflow

Hello,

On 08/25/2010 11:11 AM, Johannes Berg wrote:
> Thanks Tejun. Come to think of it, since it's an underflow it should be
> easier for me to change the BUG_ON into a printk + BUG_ON() to print out
> the current value, and then reproduce -- that way I'll know when I hit
> the situation, which isn't trivial, and also whether I hit the
> underflow. Does that sound like a good thing to try to you?

Yeap, without the fix patch applied, that should confirm that we're
seeing the same failure, and with the patch applied, you can add a
printk in the else part of !delayed check in cwq_dec_nr_in_flight().
If the printk triggers and later rmmod dosen't trigger BUG_ON(), we
can be fairly sure the problem is fixed.

Thanks.

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a2dccfc..b0d6ca4 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1702,7 +1702,9 @@ static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color,
 			if (cwq->nr_active < cwq->max_active)
 				cwq_activate_first_delayed(cwq);
 		}
-	}
+	} else
+		printk("XXX %s: cwq_dec_nr_in_flight for a delayed work\n",
+		       cwq->wq->name);

 	/* is flush in progress and are we at the flushing tip? */
 	if (likely(cwq->flush_color != color))

-- 
tejun
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