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?