On 06/10/2015 07:47 AM, 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 <kw...@redhat.com> > --- > block.c | 6 ++++++ > 1 file changed, 6 insertions(+) >
Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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