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

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.