Re: [Qemu-block] [Qemu-devel] [PATCH] block: Fix AioContext switch for bs->drv == NULL

[Eric Blake]

On 4/17/19 10:48 AM, Kevin Wolf wrote:
> Even for block nodes with bs->drv == NULL, we can't just ignore a
> bdrv_set_aio_context() call. Leaving the node in its old context can
> mean that it's still in an iothread context in bdrv_close_all() during
> shutdown, resulting in an attempted unlock of the AioContext lock which
> we don't hold.
> 

Seems like an "interesting" bug to diagnose. At any rate, getting rid of
the short-circuiting makes sense, and it's nice that you did manage to
come up with a reliable reproducer.

> The reproducer I used is a qcow2 image on gluster volume, where the
> virtual disk size (4 GB) is larger than the gluster volume size (64M),
> so we can easily trigger an ENOSPC. This backend is assigned to a
> virtio-blk device using an iothread, and then from the guest a
> 'dd if=/dev/zero of=/dev/vda bs=1G count=1' causes the VM to stop
> because of an I/O error. qemu_gluster_co_flush_to_disk() sets
> bs->drv = NULL on error, so when virtio-blk stops the dataplane, the
> block nodes stay in the iothread AioContext. A 'quit' monitor command
> issued from this paused state crashes the process.
> 
> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1631227
> Cc: address@hidden
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  block.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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