Re: [PATCH] fstests: UDEV_SETTLE_PROG before dmsetup create

Josef Bacik
2021-12-02T15:45:44Z

On Thu, Dec 02, 2021 at 02:45:24PM +0000, Filipe Manana wrote:
> On Wed, Dec 01, 2021 at 01:18:52PM -0500, Josef Bacik wrote:
> > We've been seeing transient errors with any test that uses a dm device
> > for the entirety of the time that we've been running nightly xfstests
>
> I have been having it on my tests vms since ever as well.
> It's really annoying, but fortunatelly it doesn't happen too often.
>

Yeah before this change we'd fail ~6 tests on every configruation on every
overnight run. With this change we fail 0. It's rare, but with our continual
testing it happens sooooooo much.

> > runs. This turns out to be because sometimes we get EBUSY while trying
> > to create our new dm device. Generally this is because the test comes
> > right after another test that messes with the dm device, and thus we
> > still have udev messing around with the device when DM tries to O_EXCL
> > the block device.
> >
> > Add a UDEV_SETTLE_PROG before creating the device to make sure we can
> > create our new dm device without getting this transient error.
>
> I suspect this might only make it seem the problem goes away but does not
> really fix it.
>
> I say that for 2 reasons:
>
> 1) All tests that use dm end up calling _dmsetup_remove(), like through
> _log_writes_remove() or _cleanup_flakey() for example. Normally those
> are called in the _cleanup() function, which ensures it's done even if
> the test fails for some reason.
>
> So I don't understand why we need that UDEV_SETTLE_PROG at _dmsetup_create().
>
> And I've seen the ebusy failure happen even when the previous tests did
> not use any dm device;
>
> 2) Some tests fail after creating the dm device and using it. For example
> btrfs/206 often fails when it tries to fsck the filesystem:
>
> btrfs/206 3s ... [failed, exit status 1]- output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad)
> --- tests/btrfs/206.out 2020-10-16 23:13:46.554152652 +0100
> +++ /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad 2021-12-01 21:09:46.317632589 +0000
> @@ -3,3 +3,5 @@
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> wrote 8192/8192 bytes at offset 0
> XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
> +_check_btrfs_filesystem: filesystem on /dev/sdc is inconsistent
> +(see /home/fdmanana/git/hub/xfstests/results//btrfs/206.full for details)
> ...
>
> (Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/206.out /home/fdmanana/git/hub/xfstests/results//btrfs/206.out.bad' to see the entire diff)
>
> In the .full file I got:
>
> (...)
> replaying 1239@11201: sector 2173408, size 16384, flags 0x10(METADATA)
> replaying 1240@11234: sector 0, size 0, flags 0x1(FLUSH)
> replaying 1241@11235: sector 128, size 4096, flags 0x12(FUA|METADATA)
> _check_btrfs_filesystem: filesystem on /dev/sdc is inconsistent
> *** fsck.btrfs output ***
> ERROR: cannot open device '/dev/sdc': Device or resource busy
> ERROR: cannot open file system
> Opening filesystem to check...
> *** end fsck.btrfs output
> *** mount output ***
>
> The ebusy failure is not when the test starts, but when somewhere in the middle
> of the replay loop when it calls fsck, or when it ends and the fstests framework
> calls fsck.
>
> I've seen that with btrfs/172 too, which also uses dm logwrites in a similar way.
>
> So to me this suggests 2 things:
>
> 1) Calling UDEV_SETTLE_PROG at _dmsetup_create() doesn't solve that problem with
> btrfs/206 (and other tests) - the problem is fsck failing to open the scratch
> device after it called _log_writes_remove() -> _dmsetup_remove(), and not a
> failure to create the dm device;
>
> 2) The problem is likely something missing at _dmsetup_remove(). Perhaps add
> another UDEV_SETTLE_PROG there:
>
> diff --git a/common/rc b/common/rc
> index 8e351f17..22b34677 100644
> --- a/common/rc
> +++ b/common/rc
> @@ -4563,6 +4563,7 @@ _dmsetup_remove()
> $UDEV_SETTLE_PROG >/dev/null 2>&1
> $DMSETUP_PROG remove "$@" >>$seqres.full 2>&1
> $DMSETUP_PROG mknodes >/dev/null 2>&1
> + $UDEV_SETTLE_PROG >/dev/null 2>&1
> }
>
> _dmsetup_create()
>
> I can't say if that change to _dmsetup_remove() is correct, or what it's
> needed, as I really haven't spent time trying to figure out why the issue
> happens.
>

I actually tried a few iterations before I settled on this one, but I was only
trying to reproduce the EBUSY when creating the flakey device, I hadn't seen it
with fsck. So I originally started with your change, but it didn't fix the
problem. Then I did both, UDEV_SETTLE at the end of remove and at the beginning
of create and the problem went away, and then I removed the one from remove and
the problem still was gone.

But since I've made this change I also have been investigating another problem
where we'll get EBUSY at mkfs time when we use SCRATCH_DEV_POOL. I have a test
patch in our staging branch to make sure it actuall fixes it, but I have to add
this to the start of _scratch_pool_mkfs as well.

It turns out that udev is doing this thing where it writes to
/sys/block/whatever/uevent to make sure that a KOBJ_CHANGE event gets sent out
for the device.

This is racing with the test doing a mount. So the mount gets O_EXCL, which
means the uevent doesn't get emitted until umount. This would explain what
you're seeing, we umount, we get the KOBJ_CHANGE event once the O_EXCL is
dropped, udev runs, and then fsck gets an EBUSY.

This is a very long email to say that udev is causing spurious failures because
of behavior I don't entirely understand. We're going to need to sprinkle in
UDEV_SETTLE_PROG in different places to kill all of these different scenarios.

What do we think is best here? Put UDEV_SETTLE_PROG at the start of any
function that needs to do O_EXCL? So this would mean

_dmsetup_create
_dmsetup_remove
*_mkfs
*_mount
*_check

That would be safest, and I can certainly do that. My initial approach was just
to do it where it was problematic, but the debugging I did yesterday around
btrfs/223 failures and the fact that udev is queue'ing up events that get
delivered at some point in the future makes it kind of hard to handle on a
case-by-case basis. Thanks,

Josef