From: Sergio Lopez <slp@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Fam Zheng <fam@euphon.net>,
Stefano Stabellini <sstabellini@kernel.org>,
qemu-block@nongnu.org, Paul Durrant <paul@xen.org>,
"Michael S. Tsirkin" <mst@redhat.com>,
qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
Stefan Hajnoczi <stefanha@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Anthony Perard <anthony.perard@citrix.com>,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH v2 2/4] block: Avoid processing BDS twice in bdrv_set_aio_context_ignore()
Date: Tue, 15 Dec 2020 18:23:37 +0100 [thread overview]
Message-ID: <20201215172337.w7vcn2woze2ejgco@mhamilton> (raw)
In-Reply-To: <20201215150119.GE8185@merkur.fritz.box>
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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.
> 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
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.
Sergio.
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next prev parent reply other threads:[~2020-12-15 17:25 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-14 17:05 [PATCH v2 0/4] nbd/server: Quiesce coroutines on context switch Sergio Lopez
2020-12-14 17:05 ` [PATCH v2 1/4] block: Honor blk_set_aio_context() context requirements Sergio Lopez
2020-12-15 11:58 ` Kevin Wolf
2020-12-14 17:05 ` [PATCH v2 2/4] block: Avoid processing BDS twice in bdrv_set_aio_context_ignore() Sergio Lopez
2020-12-15 12:12 ` Kevin Wolf
2020-12-15 13:15 ` Sergio Lopez
2020-12-15 15:01 ` Kevin Wolf
2020-12-15 17:23 ` Sergio Lopez [this message]
2020-12-16 12:35 ` Kevin Wolf
2020-12-16 14:55 ` Sergio Lopez
2020-12-16 18:31 ` Kevin Wolf
2020-12-17 9:37 ` Sergio Lopez
2020-12-17 10:58 ` Kevin Wolf
2020-12-17 12:50 ` Vladimir Sementsov-Ogievskiy
2020-12-17 13:06 ` Kevin Wolf
2020-12-17 13:27 ` Sergio Lopez
2020-12-17 14:01 ` Vladimir Sementsov-Ogievskiy
2020-12-17 13:09 ` Sergio Lopez
2020-12-14 17:05 ` [PATCH v2 3/4] nbd/server: Quiesce coroutines on context switch Sergio Lopez
2020-12-14 17:05 ` [PATCH v2 4/4] block: Close block exports in two steps Sergio Lopez
2020-12-15 15:34 ` Kevin Wolf
2020-12-15 17:26 ` Sergio Lopez
2020-12-21 17:07 ` Sergio Lopez
2021-01-20 20:49 ` [PATCH v2 0/4] nbd/server: Quiesce coroutines on context switch Eric Blake
2021-01-21 5:57 ` Sergio Lopez
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