Re: [Qemu-devel] [PATCH for-2.11] block: Keep strong reference when draining all BDS

[Eric Blake <eblake@redhat.com>]

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On 11/09/2017 02:43 PM, Max Reitz wrote:
> Draining a BDS may lead to graph modifications, which in turn may result
> in it and other BDS being stripped of their current references.  If
> bdrv_drain_all_begin() and bdrv_drain_all_end() do not keep strong
> references themselves, the BDS they are trying to drain (or undrain) may
> disappear right under their feet -- or, more specifically, under the
> feet of BDRV_POLL_WHILE() in bdrv_drain_recurse().
> 
> This fixes an occasional hang of iotest 194.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/io.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 44 insertions(+), 3 deletions(-)

> +
> +        /* Keep a strong reference to all root BDS and copy them into
> +         * an own list because draining them may lead to graph

'an own' sounds awkward; maybe 'copy them into a local list'

> +         * modifications. */
> +        bdrv_ref(bs);
> +        bs_list = g_slist_prepend(bs_list, bs);
>      }

>  void bdrv_drain_all_end(void)
>  {
>      BlockDriverState *bs;
>      BdrvNextIterator it;
> +    GSList *bs_list = NULL, *bs_list_entry;
> +
> +    /* Must be called from the main loop */
> +    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
>  
> +    /* Keep a strong reference to all root BDS and copy them into an
> +     * own list because draining them may lead to graph modifications.

And again.

With that tweak,
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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