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

[Sergio Lopez <slp@redhat.com>]

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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:
> > > Am 15.12.2020 um 18:23 hat Sergio Lopez geschrieben:
> > > > On Tue, Dec 15, 2020 at 04:01:19PM +0100, Kevin Wolf wrote:
> > > > > Am 15.12.2020 um 14:15 hat Sergio Lopez geschrieben:
> > > > > > On Tue, Dec 15, 2020 at 01:12:33PM +0100, Kevin Wolf wrote:
> > > > > > > Am 14.12.2020 um 18:05 hat Sergio Lopez geschrieben:
> > > > > > > > While processing the parents of a BDS, one of the parents may process
> > > > > > > > the child that's doing the tail recursion, which leads to a BDS being
> > > > > > > > processed twice. This is especially problematic for the aio_notifiers,
> > > > > > > > as they might attempt to work on both the old and the new AIO
> > > > > > > > contexts.
> > > > > > > > 
> > > > > > > > To avoid this, add the BDS pointer to the ignore list, and check the
> > > > > > > > child BDS pointer while iterating over the children.
> > > > > > > > 
> > > > > > > > Signed-off-by: Sergio Lopez <slp@redhat.com>
> > > > > > > 
> > > > > > > Ugh, so we get a mixed list of BdrvChild and BlockDriverState? :-/
> > > > > > 
> > > > > > I know, it's effective but quite ugly...
> > > > > > 
> > > > > > > What is the specific scenario where you saw this breaking? Did you have
> > > > > > > multiple BdrvChild connections between two nodes so that we would go to
> > > > > > > the parent node through one and then come back to the child node through
> > > > > > > the other?
> > > > > > 
> > > > > > I don't think this is a corner case. If the graph is walked top->down,
> > > > > > there's no problem since children are added to the ignore list before
> > > > > > getting processed, and siblings don't process each other. But, if the
> > > > > > graph is walked bottom->up, a BDS will start processing its parents
> > > > > > without adding itself to the ignore list, so there's nothing
> > > > > > preventing them from processing it again.
> > > > > 
> > > > > I don't understand. child is added to ignore before calling the parent
> > > > > callback on it, so how can we come back through the same BdrvChild?
> > > > > 
> > > > >     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);
> > > > >         child->klass->set_aio_ctx(child, new_context, ignore);
> > > > >     }
> > > > 
> > > > Perhaps I'm missing something, but the way I understand it, that loop
> > > > is adding the BdrvChild pointer of each of its parents, but not the
> > > > BdrvChild pointer of the BDS that was passed as an argument to
> > > > b_s_a_c_i.
> > > 
> > > Generally, the caller has already done that.
> > > 
> > > In the theoretical case that it was the outermost call in the recursion
> > > and it hasn't (I couldn't find any such case), I think we should still
> > > call the callback for the passed BdrvChild like we currently do.
> > > 
> > > > > You didn't dump the BdrvChild here. I think that would add some
> > > > > information on why we re-entered 0x555ee2fbf660. Maybe you can also add
> > > > > bs->drv->format_name for each node to make the scenario less abstract?
> > > > 
> > > > I've generated another trace with more data:
> > > > 
> > > > bs=0x565505e48030 (backup-top) enter
> > > > bs=0x565505e48030 (backup-top) processing children
> > > > bs=0x565505e48030 (backup-top) calling bsaci child=0x565505e42090 (child->bs=0x565505e5d420)
> > > > bs=0x565505e5d420 (qcow2) enter
> > > > bs=0x565505e5d420 (qcow2) processing children
> > > > bs=0x565505e5d420 (qcow2) calling bsaci child=0x565505e41ea0 (child->bs=0x565505e52060)
> > > > bs=0x565505e52060 (file) enter
> > > > bs=0x565505e52060 (file) processing children
> > > > bs=0x565505e52060 (file) processing parents
> > > > bs=0x565505e52060 (file) processing itself
> > > > bs=0x565505e5d420 (qcow2) processing parents
> > > > bs=0x565505e5d420 (qcow2) calling set_aio_ctx child=0x5655066a34d0
> > > > bs=0x565505fbf660 (qcow2) enter
> > > > bs=0x565505fbf660 (qcow2) processing children
> > > > bs=0x565505fbf660 (qcow2) calling bsaci child=0x565505e41d20 (child->bs=0x565506bc0c00)
> > > > bs=0x565506bc0c00 (file) enter
> > > > bs=0x565506bc0c00 (file) processing children
> > > > bs=0x565506bc0c00 (file) processing parents
> > > > bs=0x565506bc0c00 (file) processing itself
> > > > bs=0x565505fbf660 (qcow2) processing parents
> > > > bs=0x565505fbf660 (qcow2) calling set_aio_ctx child=0x565505fc7aa0
> > > > bs=0x565505fbf660 (qcow2) calling set_aio_ctx child=0x5655068b8510
> > > > bs=0x565505e48030 (backup-top) enter
> > > > bs=0x565505e48030 (backup-top) processing children
> > > > bs=0x565505e48030 (backup-top) calling bsaci child=0x565505e3c450 (child->bs=0x565505fbf660)
> > > > bs=0x565505fbf660 (qcow2) enter
> > > > bs=0x565505fbf660 (qcow2) processing children
> > > > bs=0x565505fbf660 (qcow2) processing parents
> > > > bs=0x565505fbf660 (qcow2) processing itself
> > > > bs=0x565505e48030 (backup-top) processing parents
> > > > bs=0x565505e48030 (backup-top) calling set_aio_ctx child=0x565505e402d0
> > > > bs=0x565505e48030 (backup-top) processing itself
> > > > bs=0x565505fbf660 (qcow2) processing itself
> > > 
> > > Hm, is this complete? Is see no "processing itself" for
> > > bs=0x565505e5d420. Or is this because it crashed before getting there?
> > 
> > Yes, it crashes there. I forgot to mention that, sorry.
> > 
> > > 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?

Thanks,
Sergio.

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