From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43)
	id 1JhKxy-0004V6-6A
	for qemu-devel@nongnu.org; Thu, 03 Apr 2008 04:38:38 -0400
Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43)
	id 1JhKxx-0004TT-0F
	for qemu-devel@nongnu.org; Thu, 03 Apr 2008 04:38:37 -0400
Received: from [199.232.76.173] (helo=monty-python.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1JhKxw-0004T2-Ij
	for qemu-devel@nongnu.org; Thu, 03 Apr 2008 04:38:36 -0400
Received: from bzq-179-150-194.static.bezeqint.net ([212.179.150.194]
	helo=il.qumranet.com) by monty-python.gnu.org with esmtp (Exim 4.60)
	(envelope-from <avi@qumranet.com>) id 1JhKxw-0002iE-04
	for qemu-devel@nongnu.org; Thu, 03 Apr 2008 04:38:36 -0400
Message-ID: <47F49770.1010906@qumranet.com>
Date: Thu, 03 Apr 2008 11:38:08 +0300
From: Avi Kivity <avi@qumranet.com>
MIME-Version: 1.0
Subject: Re: [Qemu-devel] QEMU/KVM SCSI lock up
References: <87k5jfixcb.fsf@fftw.org>
In-Reply-To: <87k5jfixcb.fsf@fftw.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Reply-To: qemu-devel@nongnu.org
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.gnu.org/pipermail/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Matteo Frigo <athena@fftw.org>
Cc: qemu-devel@nongnu.org

Matteo Frigo wrote:
> kvm-64 hangs under heavy disk I/O with scsi disks.  To reproduce,
> create a fresh qcow2 disk, boot linux, and execute
>
>   dd if=/dev/sdX of=/dev/null bs=1M
>
> on the fresh disk.  See also https://sourceforge.net/tracker/?func=detail&atid=893831&aid=1895893&group_id=180599
>
> I have attached a patch that appears to fix the problem.  The bug
> seems to be the following.  scsi_read_data() does the following
>
>     bdrv_aio_read()
>     r->sector += n;
>     r->sector_count -= n;
>
> For reasons that I do not fully understand, bdrv_aio_read() does
> not return immediately, but instead it calls scsi_read_data()
> recursively.

What happens (I think) is that bdrv_aio_read() completes immediately, 
calls the completion callback, which starts a read for the next batch of 
sectors.


>   Since ``r->sector += n;'' has not been executed
> yet, the re-entrant call triggers a read of the same sector, which
> breaks the producer-consumer lockstep.  The fix is to swap the operations
> as follows:
>
>     r->sector += n;
>     r->sector_count -= n;
>     bdrv_aio_read()
>
> A similar fix applies to scsi_write_data().
>
>   

Will that not issue the read for the wrong sector?

I think the correct fix is to move r->sector and r->sector_count 
adjustment into scsi_read_complete() and scsi_write_complete().

Long term we want to replace the recursion by queuing.

-- 
error compiling committee.c: too many arguments to function