For zoned block devices using zone write plugging, an rcu_barrier() call
is needed in disk_free_zone_resources() to synchronize freeing of zone
write plugs and the destrution of the mempool used to allocate the
plugs. The barrier call does slow down a little teardown of zoned block
devices but should not affect teardown of regular block devices or zoned
block devices that do not use zone write plugging (e.g. zoned DM devices
that do not require zone append emulation).

Modify disk_free_zone_resources() to return early if we do not have a
mempool to start with, that is, if the device does not use zone write
plugging. This avoids the costly rcu_barrier() and speeds up disk
teardown.
Reported-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: dd291d77 ("block: Introduce zone write plugging")
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://lore.kernel.org/r/20240607002126.104227-1-dlemoal@kernel.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>