linux-kernel - Re: NULL ptr deref at elv_drain

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Message-ID: <4EB2BE5D.5010300@suse.cz>
Date:	Thu, 03 Nov 2011 17:16:29 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Tejun Heo <tj@...nel.org>
CC:	Jens Axboe <axboe@...nel.dk>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: NULL ptr deref at elv_drain_elevator

On 11/03/2011 05:14 PM, Tejun Heo wrote:
> On Thu, Nov 03, 2011 at 05:08:40PM +0100, Jiri Slaby wrote:
>> Hi,
>>
>> I'm seeing the NULL ptr dereference below on each boot of KVM virtual
>> machine. q->elevator is NULL. This is next-20111025.
>>
>> I tried to apply Tejun's patch from:
>>  https://lkml.org/lkml/2011/4/30/87
>> but it doesn't help. Maybe I should revert something?
>>
>> Scanning for LVM volume groups...
>> Reading all physical volumes.  This may take a while...
>> No volume groups found
>> BUG: unable to handle kernel NULL pointer dereference at           (null)
>> IP: [<ffffffff8125a69c>] elv_drain_elevator+0x1c/0x70
>> PGD 46176067 PUD 452b5067 PMD 0
>> Oops: 0000 [#1] SMP
>> CPU 0
>> Modules linked in:
>>
>> Pid: 830, comm: kworker/0:2 Not tainted 3.1.0-next-20111025_64+ #1590
>> Bochs Bochs
>> RIP: 0010:[<ffffffff8125a69c>]  [<ffffffff8125a69c>]
>> elv_drain_elevator+0x1c/0x70
> 
> Heh, probably md is tearing down a queue which isn't fully setup.
> Does the following fix the problem?

Yes, it does. Thanks a lot.

> diff --git a/block/blk-core.c b/block/blk-core.c
> index f658711..5292e31 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -408,7 +408,8 @@ void blk_cleanup_queue(struct request_queue *q)
>  	mutex_unlock(&q->sysfs_lock);
>  
>  	/* drain all requests queued before DEAD marking */
> -	blk_drain_queue(q, true);
> +	if (q->elevator)
> +		blk_drain_queue(q, true);
>  
>  	/* @q won't process any more request, flush async actions */
>  	del_timer_sync(&q->backing_dev_info.laptop_mode_wb_timer);
> 
> 


-- 
js
suse labs
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