Re: [Qemu-devel] [Qemu-block] [PATCH v2 06/16] backup: Request BLK_PERM_AIO_CONTEXT_CHANGE on target

[Fam Zheng <famz@redhat.com>]

On Wed, 05/31 10:39, Stefan Hajnoczi wrote:
> On Wed, May 24, 2017 at 10:18:44AM +0800, Fam Zheng wrote:
> > On Thu, 05/11 15:41, Stefan Hajnoczi wrote:
> > > On Wed, Apr 19, 2017 at 05:43:46PM +0800, Fam Zheng wrote:
> > > > What's done in the source's context change notifier is moving the
> > > > target's context to follow the new one, so we request this permission
> > > > here.
> > > 
> > > It's true that the backup block job must be able to set target's
> > > AioContext, but does this change also allow other users to set target's
> > > AioContext while the backup job is running?  If yes, then we need to
> > > handle that.
> > 
> > If through job->target, yes, but I don't think there is any user of job->target.
> > Otherwise, it's not allowed, because the second parameter of blk_new doesn't
> > have BLK_PERM_AIO_CONTEXT_CHANGE.
> > 
> > So it's okay.
> 
> What about blockdev-backup?  It allows the user to specify 'target'.
> Therefore the user can also run other monitor commands on target.  Some
> of them could change the AioContext and the backup job wouldn't know!

That will be rejected.

The contract is that any code that wants to change the AioContext of a BDS, in
this case the "target BDS", must do this:

1) create its own BB with perm.BLK_PERM_AIO_CONTEXT_CHANGE

2) attach BDS to this BB

3) call blk_set_aio_context and change the AioContext

This is basically how all users of a BDS coordinate through Kevin's new op
blocker API, and in your concerned case, when a user runs a second monitor
command that changes AioContext, step 2 will fail, because as in this patch, the
first job->target BB didn't set shared_perm.BLK_PERM_AIO_CONTEXT_CHANGE.

Fam