From: Nikanth Karthikesan <knikanth@suse.de>
Organization: suse.de
To: Jens Axboe <jens.axboe@oracle.com>
Subject: Re: [PATCH] Exiting queue and task might race to free cic
Date: Sat, 22 Nov 2008 12:47:44 +0530
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Cc: linux-kernel@vger.kernel.org, Fabio Checconi <fchecconi@gmail.com>
References: <200811191527.18539.knikanth@suse.de> <20081119141531.GG26308@kernel.dk> <200811201027.07860.knikanth@suse.de>
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Hi Jens

On Thursday 20 November 2008 10:27:06 Nikanth Karthikesan wrote:
> 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.
>

This patch seems to solve the above said bug report on Suse kernel. So, yes it 
is reproducible! Can this be merged?

Thanks
Nikanth

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