[Qemu-devel] Re: [PATCH]: fix QEMU SCSI lock up

[Anthony Liguori <aliguori@us.ibm.com>]

Marcelo Tosatti wrote:
> From: Matteo Frigo <athena@fftw.org>
> Date: Wed, 02 Apr 2008 20:41:24 -0400
> To: qemu-devel@nongnu.org
> Subject: [Qemu-devel] QEMU/KVM SCSI lock up
> X-detected-kernel: by monty-python.gnu.org: Linux 2.6, seldom 2.4 (older, 4)
> X-Mailman-Approved-At: Thu, 03 Apr 2008 00:59:59 -0400
> Reply-To: qemu-devel@nongnu.org
>
> 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.

This bothers me.  bdrv_aio_read() should never immediately invoke the 
callback to prevent exactly this sort of problem.  Perhaps this was a 
bug that has since been fixed?  Is this still reproducible?

Regards,

Anthony Liguori