Re: [PATCH RESEND] blk-mq: order adding requests to hctx->dispatch and checking SCHED_RESTART

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 17, 2020 at 12:15:39PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 17, 2020 at 06:01:15PM +0800, Ming Lei wrote:
> > SCHED_RESTART code path is relied to re-run queue for dispatch requests
> > in hctx->dispatch. Meantime the SCHED_RSTART flag is checked when adding
> > requests to hctx->dispatch.
> > 
> > memory barriers have to be used for ordering the following two pair of OPs:
> > 
> > 1) adding requests to hctx->dispatch and checking SCHED_RESTART in
> > blk_mq_dispatch_rq_list()
> > 
> > 2) clearing SCHED_RESTART and checking if there is request in hctx->dispatch
> > in blk_mq_sched_restart().
> > 
> > Without the added memory barrier, either:
> > 
> > 1) blk_mq_sched_restart() may miss requests added to hctx->dispatch meantime
> > blk_mq_dispatch_rq_list() observes SCHED_RESTART, and not run queue in
> > dispatch side
> > 
> > or
> > 
> > 2) blk_mq_dispatch_rq_list still sees SCHED_RESTART, and not run queue
> > in dispatch side, meantime checking if there is request in
> > hctx->dispatch from blk_mq_sched_restart() is missed.
> > 
> > IO hang in ltp/fs_fill test is reported by kernel test robot:
> > 
> > 	https://lkml.org/lkml/2020/7/26/77
> > 
> > Turns out it is caused by the above out-of-order OPs. And the IO hang
> > can't be observed any more after applying this patch.
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: David Jeffery <djeffery@redhat.com>
> > Reported-by: kernel test robot <rong.a.chen@intel.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> Can you add a Fixes: tag so that the commit gets backported?

Fixes: bd166ef183c2 ("blk-mq-sched: add framework for MQ capable IO schedulers")


Thanks, 
Ming