tree 05ed9b21caf71c5d7c76af0cdae66545ba3b21a8
parent 884cb03708aadb4617a22ffd95245db97c36ca91
author Luis Chamberlain <mcgrof@kernel.org> 1586437792 +0000
committer Luis Chamberlain <mcgrof@kernel.org> 1586837052 -0600

block: revert back to synchronous request_queue removal

Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
v4.12 moved the work behind blk_release_queue() into a workqueue after a
splat floated around which indicated some work on blk_release_queue()
could sleep in blk_exit_rl(). This splat would be possible when a driver
called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
as its final call) from an atomic context.

blk_put_queue() decrements the refcount for the request_queue
kobject, and upon reaching 0 blk_release_queue() is called. Although
blk_exit_rl() is now removed through commit db6d9952356 ("block: remove
request_list code"), we reserve the right to be able to sleep within
blk_release_queue() context. If you see no other way and *have* be
in atomic context when you driver calls the last blk_put_queue()
you can always just increase your block device's reference count with
bdgrab() as this can be done in atomic context and the request_queue
removal would be left to upper layers later. We document this bit of
tribal knowledge as well now, and adjust kdoc format a bit.

We revert back to synchronous request_queue removal because asynchronous
removal creates a regression with expected userspace interaction with
several drivers. An example is when removing the loopback driver and
issues ioctl from userspace to do so, upon return and if successful one
expects the device to be removed. Moving to asynchronous request_queue
removal could have broken many scripts which relied on the removal to
have been completed if there was no error.

Using asynchronous request_queue removal however has helped us find
other bugs, in the future we can test what could break with this
arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: yu kuai <yukuai3@huawei.com>
Suggested-by: Nicolai Stange <nstange@suse.de>
Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>