Re: [Qemu-devel] [PATCH v2 2/2] blockjob: use blk_new_pinned in block_job_create

[Kevin Wolf <kwolf@redhat.com>]

Am 06.06.2019 um 14:29 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 06.06.2019 13:05, Kevin Wolf wrote:
> > Am 05.06.2019 um 19:16 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >> 05.06.2019 20:11, Kevin Wolf wrote:
> >>> Am 05.06.2019 um 14:32 hat Vladimir Sementsov-Ogievskiy geschrieben:
> >>>> child_role job already has .stay_at_node=true, so on bdrv_replace_node
> >>>> operation these child are unchanged. Make block job blk behave in same
> >>>> manner, to avoid inconsistent intermediate graph states and workarounds
> >>>> like in mirror.
> >>>>
> >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> >>>
> >>> This feels dangerous. It does what you want it to do if the only graph
> >>> change below the BlockBackend is the one in mirror_exit_common. But the
> >>> user could also take a snapshot, or in the future hopefully insert a
> >>> filter node, and you would then want the BlockBackend to move.
> >>>
> >>> To be honest, even BdrvChildRole.stay_at_node is a bit of a hack. But at
> >>> least it's only used for permissions and not for the actual data flow.
> >>
> >> Hmm. Than, may be just add a parameter to bdrv_replace_node, which parents
> >> to ignore? Would it work?
> > 
> > I would have to think a bit more about it, but it does sound safer.
> > 
> > Or we take a step back and ask why it's even a problem for the mirror
> > block job if the BlockBackend is moved to a different node. The main
> > reason I see is because of bs->job that is set for the root node of the
> > BlockBackend and needs to be unset for the same node.
> > 
> > Maybe we can just finally get rid of bs->job? It doesn't have many users
> > any more.
> > 
> 
> Hmm, looked at it. Not sure what should be refactored around job to get rid
> of "main node" concept.. Which seems to be in a bad relation with starting
> job on implicit filters as a main node..
> 
> But about just removing bs->job pointer, I don't know at least what to do with
> blk_iostatus_reset and blockdev_mark_auto_del..

blk_iostatus_reset() looks easy. It has only two callers:

1. blk_attach_dev(). This doesn't have anything to do with jobs and
   attaching a new guest device won't solve any problem the job
   encountered, so no reason to reset the iostatus for the job.

2. qmp_cont(). This resets the iostatus for everything. We can just
   call block_job_iostatus_reset() for all block jobs instead of going
   through BlockBackend.

blockdev_mark_auto_del() might be a bit trickier. The whole idea of the
function is: When a guest device gets unplugged, automatically remove
its root block node, too. Commit 12bde0eed6b made it cancel a block job
because that should happen immediately when the device is actually
released by the guest and not only after the job finishes and gives up
its reference. I would like to just change the behaviour, but I'm afraid
we can't do this because of compatibility.

However, just checking bs->job is really only one special case of
another user of the node to be deleted. Maybe we can extend it a little
so that any block jobs that contain the node in job->nodes are
cancelled.

Kevin