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
