On Wed, Apr 29, 2015 at 5:32 PM, NeilBrown [off-list ref] wrote:

bdi_unregister() now contains very little functionality.

It contains a "WARN_ON" if bdi->dev is NULL.  This warning is of no
real consequence as bdi->dev isn't needed by anything else in the function,
and it triggers if
   blk_cleanup_queue() -> bdi_destroy()
is called before bdi_unregister, which a subsequent patch will make happen.
So this isn't wanted.

It also calls bdi_set_min_ratio().  This needs to be called after
writes through the bdi have all been flushed, and before the bdi is destroyed.
Calling it early is better than calling it late as it frees up a global
resource.

Calling it immediately after bdi_wb_shutdown() in bdi_destroy()
perfectly fits these requirements.

So bdi_unregister can be discarded with the important content moved to
bdi_destroy, as can the
  writeback_bdi_unregister
event which is already not used.

This is tagged for 'stable' as it is a pre-requisite for a subsequent
patch which moves calls to blk_cleanup_queue() before calls to
del_gendisk().  The commit identified as 'Fixes' removed a lot of
other functionality from bdi_unregister(), and made a change which
necessitated moving the blk_cleanup_queue() calls.

Reported-by: Mike Snitzer <redacted>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org (v4.0)
Fixes: c4db59d31e39ea067c32163ac961e9c80198fd37
Signed-off-by: NeilBrown <redacted>

---

Hi again Jens,
 would you be able to queue this patch *before* the other one:
   block: destroy bdi before blockdev is unregistered.

 If it has to come after I'll need to re-write the text a bit.
 If you could give me the commit hash to reference I'll do that.

Seems it is after:

http://git.kernel.dk/?p=linux-block.git;a=commit;h=6cd18e71

Also, we gave both patches a try internally after seeing the duplicate
sysfs warning.  You can add:

Acked-by: Dan Williams <redacted>
Tested-by: Nicholas Moulin <redacted>

...on the re-send.

Thanks Neil!