linux-kernel - Re: [PATCH] Exiting queue and task might race to free cic

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Message-Id: <200811201027.07860.knikanth@suse.de>
Date:	Thu, 20 Nov 2008 10:27:06 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, Fabio Checconi <fchecconi@...il.com>
Subject: Re: [PATCH] Exiting queue and task might race to free cic

On Wednesday 19 November 2008 19:45:31 Jens Axboe wrote:
> On Wed, Nov 19 2008, Nikanth Karthikesan wrote:

> Not sure this is enough, we probably need to copy the key to ensure that
> we get a fresh value. How does this look?
>

Agreed. Read barrier required. But the compiler hint, "likely" can stay?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6a062ee..4504b94 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1318,7 +1318,15 @@ static void cfq_exit_single_io_context(struct 
io_context *ioc,
 		unsigned long flags;
 
 		spin_lock_irqsave(q->queue_lock, flags);
-		__cfq_exit_single_io_context(cfqd, cic);
+
+		/*
+		 * Ensure we get a fresh copy of the ->key to prevent
+		 * race between exiting task and queue
+		 */
+		smp_read_barrier_depends();
+		if (likely(cic->key))
+			__cfq_exit_single_io_context(cfqd, cic);
+
 		spin_unlock_irqrestore(q->queue_lock, flags);
 	}
 }


> Did you actually trigger this, or is it just from code inspection?
>

No. But I am looking at another bug report on Suse Kernel where the bug is 
triggered during reboot when the kernel thread usb_stor_scan_thread exits.

Thanks
Nikanth Karthikesan
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