Re: [Qemu-devel] [PATCH v2 2/3] block/fleecing-filter: new filter driver for fleecing

[Fam Zheng <famz@redhat.com>]

On Fri, 06/29 12:24, Eric Blake wrote:
> On 06/29/2018 10:15 AM, Vladimir Sementsov-Ogievskiy wrote:
> > We need to synchronize backup job with reading from fleecing image
> > like it was done in block/replication.c.
> > 
> > Otherwise, the following situation is theoretically possible:
> > 
> 
> Grammar suggestions:
> 
> > 1. client start reading
> 
> client starts reading
> 
> > 2. client understand, that there is no corresponding cluster in
> >     fleecing image
> > 3. client is going to read from backing file (i.e. active image)
> 
> client sees that no corresponding cluster has been allocated in the fleecing
> image, so the request is forwarded to the backing file
> 
> > 4. guest writes to active image
> > 5. this write is stopped by backup(sync=none) and cluster is copied to
> >     fleecing image
> > 6. guest write continues...
> > 7. and client reads _new_ (or partly new) date from active image
> 
> Interesting race. Can it actually happen, or does our read code already
> serialize writes to the same area while a read is underway?

Yes, I wonder why wait_serialising_requests() is not enough. If it's possible,
can we have a test case (with help of blkdebug, for example)?

> 
> In short, I see what problem you are claiming exists: the moment the client
> starts reading from the backing file, that portion of the backing file must
> remain unchanged until after the client is done reading.  But I don't know
> enough details of the block layer to know if this is actually a problem, or
> if adding the new filter is just overhead.

Fam