Re: [PATCH] btrfs: test for device flush-able should be after wait code

[David Sterba <dsterba@suse.cz>]

On Wed, Jun 14, 2017 at 06:02:38PM +0800, Anand Jain wrote:
> write_dev_flush() checks if the device is flush capable, however as
> the device mode can change any time, this check should be after the
> wait part of the code.

This would apply to code before "btrfs: wait part of the
write_dev_flush() can be separated out", right?

The problematic case is when a flush bio is sent, queue status changes
and waiting is skipped. A bio would leak. Next time the flushing is
enabled, write_dev_flush allocaes a new bio and either it would leak or
waiting will happen as expected.

The bio would leak only if the barriers are switched between write and
wait. Not impossible, but I still think a race hard to win. The
consequences are not absolutely fatal.

Also, the cleanups in write_dev_flush fix the bug in another way, so we
don't need this separate patch as a potential stable backport. The patch
mentioned above can be considered a fix but would need some manual
adaptations to apply. Therefore I don't think we need the $subj patch.