Re: [PATCH v2 2/4] block: Avoid processing BDS twice in bdrv_set_aio_context_ignore()

[Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>]

17.12.2020 16:06, Kevin Wolf wrote:
> Am 17.12.2020 um 13:50 hat Vladimir Sementsov-Ogievskiy geschrieben:
> > 17.12.2020 13:58, Kevin Wolf wrote:
> > > Am 17.12.2020 um 10:37 hat Sergio Lopez geschrieben:
> > > > On Wed, Dec 16, 2020 at 07:31:02PM +0100, Kevin Wolf wrote:
> > > > > Am 16.12.2020 um 15:55 hat Sergio Lopez geschrieben:
> > > > > > On Wed, Dec 16, 2020 at 01:35:14PM +0100, Kevin Wolf wrote:
> > > > > > > Anyway, trying to reconstruct the block graph with BdrvChild pointers
> > > > > > > annotated at the edges:
> > > > > > >
> > > > > > > BlockBackend
> > > > > > >         |
> > > > > > >         v
> > > > > > >     backup-top ------------------------+
> > > > > > >         |   |                          |
> > > > > > >         |   +-----------------------+  |
> > > > > > >         |            0x5655068b8510 |  | 0x565505e3c450
> > > > > > >         |                           |  |
> > > > > > >         | 0x565505e42090            |  |
> > > > > > >         v                           |  |
> > > > > > >       qcow2 ---------------------+  |  |
> > > > > > >         |                        |  |  |
> > > > > > >         | 0x565505e52060         |  |  | ??? [1]
> > > > > > >         |                        |  |  |  |
> > > > > > >         v         0x5655066a34d0 |  |  |  | 0x565505fc7aa0
> > > > > > >       file                       v  v  v  v
> > > > > > >                                qcow2 (backing)
> > > > > > >                                       |
> > > > > > >                                       | 0x565505e41d20
> > > > > > >                                       v
> > > > > > >                                     file
> > > > > > >
> > > > > > > [1] This seems to be a BdrvChild with a non-BDS parent. Probably a
> > > > > > >       BdrvChild directly owned by the backup job.
> > > > > > >
> > > > > > > > So it seems this is happening:
> > > > > > > >
> > > > > > > > backup-top (5e48030) <---------| (5)
> > > > > > > >      |    |                      |
> > > > > > > >      |    | (6) ------------> qcow2 (5fbf660)
> > > > > > > >      |                           ^    |
> > > > > > > >      |                       (3) |    | (4)
> > > > > > > >      |-> (1) qcow2 (5e5d420) -----    |-> file (6bc0c00)
> > > > > > > >      |
> > > > > > > >      |-> (2) file (5e52060)
> > > > > > > >
> > > > > > > > backup-top (5e48030), the BDS that was passed as argument in the first
> > > > > > > > bdrv_set_aio_context_ignore() call, is re-entered when qcow2 (5fbf660)
> > > > > > > > is processing its parents, and the latter is also re-entered when the
> > > > > > > > first one starts processing its children again.
> > > > > > > Yes, but look at the BdrvChild pointers, it is through different edges
> > > > > > > that we come back to the same node. No BdrvChild is used twice.
> > > > > > >
> > > > > > > If backup-top had added all of its children to the ignore list before
> > > > > > > calling into the overlay qcow2, the backing qcow2 wouldn't eventually
> > > > > > > have called back into backup-top.
> > > > > > I've tested a patch that first adds every child to the ignore list,
> > > > > > and then processes those that weren't there before, as you suggested
> > > > > > on a previous email. With that, the offending qcow2 is not re-entered,
> > > > > > so we avoid the crash, but backup-top is still entered twice:
> > > > > I think we also need to every parent to the ignore list before calling
> > > > > callbacks, though it doesn't look like this is the problem you're
> > > > > currently seeing.
> > > > I agree.
> > > >
> > > > > > bs=0x560db0e3b030 (backup-top) enter
> > > > > > bs=0x560db0e3b030 (backup-top) processing children
> > > > > > bs=0x560db0e3b030 (backup-top) calling bsaci child=0x560db0e2f450 
> > > > > > (child->bs=0x560db0fb2660)
> > > > > > bs=0x560db0fb2660 (qcow2) enter
> > > > > > bs=0x560db0fb2660 (qcow2) processing children
> > > > > > bs=0x560db0fb2660 (qcow2) calling bsaci child=0x560db0e34d20 
> > > > > > (child->bs=0x560db1bb3c00)
> > > > > > bs=0x560db1bb3c00 (file) enter
> > > > > > bs=0x560db1bb3c00 (file) processing children
> > > > > > bs=0x560db1bb3c00 (file) processing parents
> > > > > > bs=0x560db1bb3c00 (file) processing itself
> > > > > > bs=0x560db0fb2660 (qcow2) calling bsaci child=0x560db16964d0 
> > > > > > (child->bs=0x560db0e50420)
> > > > > > bs=0x560db0e50420 (qcow2) enter
> > > > > > bs=0x560db0e50420 (qcow2) processing children
> > > > > > bs=0x560db0e50420 (qcow2) calling bsaci child=0x560db0e34ea0 
> > > > > > (child->bs=0x560db0e45060)
> > > > > > bs=0x560db0e45060 (file) enter
> > > > > > bs=0x560db0e45060 (file) processing children
> > > > > > bs=0x560db0e45060 (file) processing parents
> > > > > > bs=0x560db0e45060 (file) processing itself
> > > > > > bs=0x560db0e50420 (qcow2) processing parents
> > > > > > bs=0x560db0e50420 (qcow2) processing itself
> > > > > > bs=0x560db0fb2660 (qcow2) processing parents
> > > > > > bs=0x560db0fb2660 (qcow2) calling set_aio_ctx child=0x560db1672860
> > > > > > bs=0x560db0fb2660 (qcow2) calling set_aio_ctx child=0x560db1b14a20
> > > > > > bs=0x560db0e3b030 (backup-top) enter
> > > > > > bs=0x560db0e3b030 (backup-top) processing children
> > > > > > bs=0x560db0e3b030 (backup-top) processing parents
> > > > > > bs=0x560db0e3b030 (backup-top) calling set_aio_ctx child=0x560db0e332d0
> > > > > > bs=0x560db0e3b030 (backup-top) processing itself
> > > > > > bs=0x560db0fb2660 (qcow2) processing itself
> > > > > > bs=0x560db0e3b030 (backup-top) calling bsaci child=0x560db0e35090 
> > > > > > (child->bs=0x560db0e50420)
> > > > > > bs=0x560db0e50420 (qcow2) enter
> > > > > > bs=0x560db0e3b030 (backup-top) processing parents
> > > > > > bs=0x560db0e3b030 (backup-top) processing itself
> > > > > >
> > > > > > I see that "blk_do_set_aio_context()" passes "blk->root" to
> > > > > > "bdrv_child_try_set_aio_context()" so it's already in the ignore list,
> > > > > > so I'm not sure what's happening here. Is backup-top is referenced
> > > > > > from two different BdrvChild or is "blk->root" not pointing to
> > > > > > backup-top's BDS?
> > > > > The second time that backup-top is entered, it is not as the BDS of
> > > > > blk->root, but as the parent node of the overlay qcow2. Which is
> > > > > interesting, because last time it was still the backing qcow2, so the
> > > > > change did have _some_ effect.
> > > > >
> > > > > The part that I don't understand is why you still get the line with
> > > > > child=0x560db1b14a20, because when you add all children to the ignore
> > > > > list first, that should have been put into the ignore list as one of the
> > > > > first things in the whole process (when backup-top was first entered).
> > > > >
> > > > > Is 0x560db1b14a20 a BdrvChild that has backup-top as its opaque value,
> > > > > but isn't actually present in backup-top's bs->children?
> > > > Exactly, that line corresponds to this chunk of code:
> > > >
> > > > <---- begin ---->
> > > >       QLIST_FOREACH(child, &bs->parents, next_parent) {
> > > >           if (g_slist_find(*ignore, child)) {
> > > >               continue;
> > > >           }
> > > >           assert(child->klass->set_aio_ctx);
> > > >           *ignore = g_slist_prepend(*ignore, child);
> > > >           fprintf(stderr, "bs=%p (%s) calling set_aio_ctx child=%p\n", bs, 
> > > > bs->drv->format_name, child);
> > > >           child->klass->set_aio_ctx(child, new_context, ignore);
> > > >       }
> > > > <---- end ---->
> > > >
> > > > Do you think it's safe to re-enter backup-top, or should we look for a
> > > > way to avoid this?
> > > I think it should be avoided, but I don't understand why putting all
> > > children of backup-top into the ignore list doesn't already avoid it. If
> > > backup-top is in the parents list of qcow2, then qcow2 should be in the
> > > children list of backup-top and therefore the BdrvChild should already
> > > be in the ignore list.
> > >
> > > The only way I can explain this is that backup-top and qcow2 have
> > > different ideas about which BdrvChild objects exist that connect them.
> > > Or that the graph changes between both places, but I don't see how that
> > > could happen in bdrv_set_aio_context_ignore().
> > bdrv_set_aio_context_ignore() do bdrv_drained_begin().. As I reported
> > recently, nothing prevents some job finish and do graph modification
> > during some another drained section. It may be the case.
> Good point, this might be the same bug then.
>
> If everything worked correctly, a job completion could only happen on
> the outer bdrv_set_aio_context_ignore(). But after that, we are already
> in a drain section, so the job should be quiesced and a second drain
> shouldn't cause any additional graph changes.
>
> I would have to go back to the other discussion, but I think it was
> related to block jobs that are already in the completion process and
> keep moving forward even though they are supposed to be quiesced.
>
> If I remember correctly, actually pausing them at this point looked
> difficult. Maybe what we should then do is letting .drained_poll return
> true until they have actually fully completed?
>
> Ah, but was this something that would deadlock because the job
> completion callbacks use drain sections themselves?
Hmm..  I've recently sent good example of dead-lock in email "aio-poll 
dead-lock"..

I don't have better idea than moving all graph modifications (together with
corresponding drained sections) into coroutines and protect by global coroutine
mutex.

> > If backup-top involved, I can suppose that graph modification is in
> > backup_clean, when we remove the filter.. Who is making
> > set_aio_context in the issue? I mean, what is the backtrace of
> > bdrv_set_aio_context_ignore()?
> Sergio, can you provide the backtrace and also test if the theory with a
> job completion in the middle of the process is what you actually hit?
>
> Kevin

--
Best regards,
Vladimir