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

[Jens Axboe <axboe@kernel.dk>]

On 8/17/20 3:01 AM, 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.

Applied, thanks.

-- 
Jens Axboe