Re: [Qemu-devel] [PATCH v2 2/3] mirror: Keep mirror_top_bs drained after dropping permissions

[Max Reitz <mreitz@redhat.com>]

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On 07.08.19 16:46, Kevin Wolf wrote:
> mirror_top_bs is currently implicitly drained through its connection to
> the source or the target node. However, the drain section for target_bs
> ends early after moving mirror_top_bs from src to target_bs, so that
> requests can already be restarted while mirror_top_bs is still present
> in the chain, but has dropped all permissions and therefore runs into an
> assertion failure like this:
> 
>     qemu-system-x86_64: block/io.c:1634: bdrv_co_write_req_prepare:
>     Assertion `child->perm & BLK_PERM_WRITE' failed.
> 
> Keep mirror_top_bs drained until all graph changes have completed.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block/mirror.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 9f5c59ece1..642d6570cc 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -656,7 +656,10 @@ static int mirror_exit_common(Job *job)
>      s->target = NULL;
>  
>      /* We don't access the source any more. Dropping any WRITE/RESIZE is
> -     * required before it could become a backing file of target_bs. */
> +     * required before it could become a backing file of target_bs. Not having
> +     * these permissions any more means that we can't allow any new requests on
> +     * mirror_top_bs from now on, so keep it drained. */

You’re lucky Patchew broke or it would have complained about multi-line
comments needing /* and */ on separate lines. :-)

I’m perfectly happy with this style, though:

Reviewed-by: Max Reitz <mreitz@redhat.com>

> +    bdrv_drained_begin(mirror_top_bs);
>      bs_opaque->stop = true;
>      bdrv_child_refresh_perms(mirror_top_bs, mirror_top_bs->backing,
>                               &error_abort);
> @@ -724,6 +727,7 @@ static int mirror_exit_common(Job *job)
>      bs_opaque->job = NULL;
>  
>      bdrv_drained_end(src);
> +    bdrv_drained_end(mirror_top_bs);
>      s->in_drain = false;
>      bdrv_unref(mirror_top_bs);
>      bdrv_unref(src);
> 



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