Re: [Qemu-devel] [PATCH v2 09/13] block: Drain requests before swapping nodes in bdrv_swap()

[Max Reitz <mreitz@redhat.com>]

On 10.06.2015 15:47, Kevin Wolf wrote:
> bdrv_swap() requires that there are no requests in flight on either of
> the two devices. The request coroutine would work on the wrong
> BlockDriverState object (with bs->opaque even being interpreted as a
> different type potentially) and all sorts of bad things would result
> from this.
>
> The currently existing callers mostly ensure that there is no I/O
> pending on nodes that are swapped. In detail, this is:
>
> 1. Live snapshots. This goes through qmp_transaction(), which calls
>     bdrv_drain_all() before doing anything. The command is executed
>     synchronously, so no new I/O can be issued concurrently.
>
> 2. snapshot=on in bdrv_open(). We're in the middle of opening the image
>     (both the original image and its temporary overlay), so there can't
>     be any I/O in flight yet.
>
> 3. Mirroring. bdrv_drain() is already used on the source device so that
>     the mirror doesn't miss anything. However, the main loop runs between
>     that and the bdrv_swap() (which is actually a bug, being addressed in
>     another series), so there is a small window in which new I/O might be
>     issued that would be in flight during bdrv_swap().
>
> It is safer to just drain the request queue of both devices in
> bdrv_swap() instead of relying on callers to do the right thing.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>   block.c | 6 ++++++
>   1 file changed, 6 insertions(+)

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