Re: [Qemu-devel] [PATCH 2/2] ide/atapi: partially avoid deadlock if the storage backend is dead

[Eric Blake <eblake@redhat.com>]

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On 08/20/2015 01:14 AM, Peter Lieven wrote:
> the blk_drain_all() that is executed if the guest issues a DMA cancel
> leads to a stuck main loop if the storage backend (e.g. a NFS share)
> is unresponsive.
> 
> This scenario is a common case for CDROM images mounted from an
> NFS share. In this case a broken NFS server can take down the
> whole VM even if the mounted CDROM is not used and was just not
> unmounted after usage.
> 
> This approach avoids the blk_drain_all for read-only media and
> cancelles the AIO locally and makes the callback a NOP if the

s/cancelles/cancels/

> original request is completed after the NFS share is responsive
> again.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  hw/ide/pci.c | 32 ++++++++++++++++++--------------
>  1 file changed, 18 insertions(+), 14 deletions(-)

> +                if (!bdrv_is_read_only(bm->bus->dma->aiocb->bs)) {
> +                    /* We can't cancel Scatter Gather DMA in the middle of the
> +                     * operation or a partial (not full) DMA transfer would
> +                     * reach the storage so we wait for completion instead
> +                     * (we beahve like if the DMA was completed by the time the

s/beahve like/behave as/

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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