Re: Flaky test: generic/085

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jun 11, 2024 at 09:52:10AM +0100, Theodore Ts'o wrote:
> Hi, I've recently found a flaky test, generic/085 on 6.10-rc2 and
> fs-next.  It's failing on both ext4 and xfs, and it reproduces more
> easiy with the dax config:
> 
> xfs/4k: 20 tests, 1 failures, 137 seconds
>   Flaky: generic/085:  5% (1/20)
> xfs/dax: 20 tests, 11 failures, 71 seconds
>   Flaky: generic/085: 55% (11/20)
> ext4/4k: 20 tests, 111 seconds
> ext4/dax: 20 tests, 8 failures, 69 seconds
>   Flaky: generic/085: 40% (8/20)
> Totals: 80 tests, 0 skipped, 20 failures, 0 errors, 388s
> 
> The failure is caused by a WARN_ON in fs_bdev_thaw() in fs/super.c:
> 
> static int fs_bdev_thaw(struct block_device *bdev)
> {
> 	...
> 	sb = get_bdev_super(bdev);
> 	if (WARN_ON_ONCE(!sb))
> 		return -EINVAL;
> 
> 
> The generic/085 test which exercises races between the fs
> freeze/unfeeze and mount/umount code paths, so this appears to be
> either a VFS-level or block device layer bug.  Modulo the warning, it
> looks relatively harmless, so I'll just exclude generic/085 from my
> test appliance, at least for now.  Hopefully someone will have a
> chance to take a look at it?

I think this can happen if fs_bdev_thaw races with unmount?

Let's say that the _umount $lvdev in the second loop in generic/085
starts the unmount process, which clears SB_ACTIVE from the super_block.
Then the first loop tries to freeze the bdev (and fails), and
immediately tries to thaw the bdev.  The thaw code calls fs_bdev_thaw
because the unmount process is still running & so the fs is still
holding the bdev.  But get_bdev_super sees that SB_ACTIVE has been
cleared from the super_block so it returns NULL, which trips the
warning.

If that's correct, then I think the WARN_ON_ONCE should go away.

--D

> Thanks,
> 
> 					- Ted
>