Re: [PATCH 05/14] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Tue, 2017-08-01 at 18:44 +0800, Ming Lei wrote:
> On Mon, Jul 31, 2017 at 11:42:21PM +0000, Bart Van Assche wrote:
> > Since setting, clearing and testing of BLK_MQ_S_BUSY can happen concurrently
> > and since clearing and testing happens without any locks held I'm afraid this
> > patch introduces the following race conditions:
> > [ ... ]
> > * Checking BLK_MQ_S_BUSY after requests have been removed from the dispatch list
> >   but before that bit is cleared, resulting in test_bit(BLK_MQ_S_BUSY, &hctx->state)
> >   reporting that the BLK_MQ_S_BUSY
> > has been set although there are no requests
> >   on the dispatch list.
> 
> That won't be a problem, because dispatch will be started in the
> context in which dispatch list is flushed, since the BUSY bit
> is cleared after blk_mq_dispatch_rq_list() returns. So no I/O
> hang.

Hello Ming,

Please consider changing the name of the BLK_MQ_S_BUSY constant. That bit
is used to serialize dispatching requests from the hctx dispatch list but
that's not clear from the name of that constant.

Thanks,

Bart.