md: fix a plug/unplug race in raid5
When a device is unplugged, requests are moved from one or two (depending on whether a bitmap is in use) queues to the main request queue. So whenever requests are put on either of those queues, we should make sure the raid5 array is 'plugged'. However we don't. We currently plug the raid5 queue just before putting requests on queues, so there is room for a race. If something unplugs the queue at just the wrong time, requests will be left on the queue and nothing will want to unplug them. Normally something else will plug and unplug the queue fairly soon, but there is a risk that nothing will. Signed-off-by: Neil Brown <neilb@suse.de> Signed-off-by: Adrian Bunk <bunk@kernel.org>
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