Am 08.04.2016 um 17:49 hat Max Reitz geschrieben: > On 07.04.2016 13:29, Kevin Wolf wrote: > > Am 06.04.2016 um 19:57 hat Max Reitz geschrieben: > >> If bdrv_open_inherit() creates a snapshot BDS and *pbs is NULL, that > >> snapshot BDS should be returned instead of the BDS under it. > >> > >> To this end, bdrv_append_temp_snapshot() now returns the snapshot BDS > >> instead of just appending it on top of the snapshotted BDS. Also, it > >> calls bdrv_ref() before bdrv_append() (which bdrv_open_inherit() has to > >> undo if not returning the overlay). > >> > >> Signed-off-by: Max Reitz <mre...@redhat.com> > > > > This is a tricky patch, but after all it looks correct to me. I think we > > could improve a bit on the documentation, though: > > > > 1. The commit message suggests that by returning the wrong BDS we may > > have an observable bug. It would be good to add details on why this > > used to be harmless (IIUC, all users of BDRV_O_SNAPSHOT go through > > blk_new_open(), and there first setting *pbs (which is blk->root->bs) > > and then doing bdrv_append() does the right thing) > > To be honest, I'd rather not wrap my head around why it worked, but I'll > try to.
I hope you just need to verify my theory stated above. ;-)
> > 2. The refcounting stuff isn't obvious either:
> >
> >> @@ -1481,12 +1482,16 @@ static int
> >> bdrv_append_temp_snapshot(BlockDriverState *bs, int flags,
> >> goto out;
> >> }
> >>
> >> + bdrv_ref(bs_snapshot);
> >> bdrv_append(bs_snapshot, bs);
> >
> > This is because bdrv_append() drops the reference, but we want to return
> > a strong reference.
>
> Well, it's mostly because now we do return a (strong) reference. Before,
> bs_snapshot simply was not returned at all.
>
> I'll add a comment, though.
Yes, that's the additional reference you need. I just kept all existing
references in mind:
We already get a strong reference from bdrv_new(). bdrv_append() takes
ownership of a strong reference, though, and this is why we need a
second one to return.
> >> /* For snapshot=on, create a temporary qcow2 overlay. bs points to the
> >> * temporary snapshot afterwards. */
> >> if (snapshot_flags) {
> >> - ret = bdrv_append_temp_snapshot(bs, snapshot_flags,
> >> snapshot_options,
> >> - &local_err);
> >> + BlockDriverState *snapshot_bs;
> >> + snapshot_bs = bdrv_append_temp_snapshot(bs, snapshot_flags,
> >> + snapshot_options,
> >> &local_err);
> >> snapshot_options = NULL;
> >> if (local_err) {
> >> + ret = -EINVAL;
> >> goto close_and_fail;
> >> }
> >> + if (!*pbs) {
> >> + /* The reference is now held by the overlay BDS */
> >> + bdrv_unref(bs);
> >
> > We still hold a strong reference to the newly created bs that we wanted
> > to return, but now we're returning a different BDS, so we must drop the
> > reference. (The overlay BDS doesn't hold "the" same reference as the
> > comment suggests, but an additional one.)
> >
> >> + bs = snapshot_bs;
> >> + } else {
> >> + /* It is still referenced in the same way that *pbs was
> >> referenced,
> >> + * however that may be */
> >> + bdrv_unref(snapshot_bs);
> >
> > In this case we don't in fact return the reference for bs_snapshot, so
> > drop it.
> >
> > So I think what I would like here is comments that explain where the
> > ownership of the individual strong references goes, not who else may or
> > may not hold additional references to a BDS.
>
> Well, the ownership goes away. ;-)
Still good to know that this function owns the reference before it drops
it. ;-)
> I've fattened the comments so they explain exactly why the refcount is
> decremented, respectively, and why this will most likely not result in
> the deletion of the BDS.
Sounds good.
Kevin
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