linux-kernel - Re: [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for request-based dm

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Message-ID: <4C7FA531.7060300@kernel.org>
Date:	Thu, 02 Sep 2010 15:22:57 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
CC:	Mike Snitzer <snitzer@...hat.com>, jaxboe@...ionio.com,
	j-nomura@...jp.nec.com, jamie@...reable.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-raid@...r.kernel.org, hch@....de
Subject: Re: [PATCH UPDATED 4/5] dm: implement REQ_FLUSH/FUA support for request-based
 dm

Hello,

On 09/01/2010 09:15 AM, Kiyoshi Ueda wrote:
> I don't see any obvious problem on this patch.
> However, I hit a NULL pointer dereference below when I use a mpath
> device with barrier option of ext3.  I'm investigating the cause now.
> (Also I'm not sure the cause of the hang which Mike is hitting yet.)
> 
> I tried on the commit 28dd53b26d362c16234249bad61db8cbd9222d0b of
> git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git flush-fua.
> 
>     # mke2fs -j /dev/mapper/mpatha
>     # mount -o barrier=1 /dev/mapper/mpatha /mnt/0
>     # dd if=/dev/zero of=/mnt/0/a bs=512 count=1
>     # sync

Hmm... I'm trying to reproduce this problem but hasn't been successful
yet.

> BUG: unable to handle kernel NULL pointer dereference at 0000000000000078
> IP: [<ffffffffa0070ec3>] scsi_finish_command+0xa3/0x120 [scsi_mod]

Can you please ask gdb which source line this is?

> Also, I have one comment below on this patch.
> 
>> @@ -2619,9 +2458,8 @@ int dm_suspend(struct mapped_device *md,
>>  	up_write(&md->io_lock);
>>
>>  	/*
>> -	 * Request-based dm uses md->wq for barrier (dm_rq_barrier_work) which
>> -	 * can be kicked until md->queue is stopped.  So stop md->queue before
>> -	 * flushing md->wq.
>> +	 * Stop md->queue before flushing md->wq in case request-based
>> +	 * dm defers requests to md->wq from md->queue.
>>  	 */
>>  	if (dm_request_based(md))
>>  		stop_queue(md->queue);
> 
> Request-based dm doesn't use md->wq now, so you can just remove
> the comment above.

I sure can remove it but md->wq already has most stuff necessary to
process deferred requests and when someone starts using it, having the
comment there about the rather delicate ordering would definitely be
helpful, so I suggest keeping the comment.

Thanks.

-- 
tejun

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