blk-mq: order adding requests to hctx->dispatch and checking SCHED_RESTART
commit5.4-rM2-2.2.x-imx-squashedd7d8535f37
upstream. 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. Fixes:bd166ef183
("blk-mq-sched: add framework for MQ capable IO schedulers") Reported-by: kernel test robot <rong.a.chen@intel.com> Signed-off-by: Ming Lei <ming.lei@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Cc: Bart Van Assche <bvanassche@acm.org> Cc: Christoph Hellwig <hch@lst.de> Cc: David Jeffery <djeffery@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: Jens Axboe <axboe@kernel.dk> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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c2035d1e55
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b1a83ee0cb
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@ -77,6 +77,15 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
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return;
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return;
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clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
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clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
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/*
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* Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
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* in blk_mq_run_hw_queue(). Its pair is the barrier in
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* blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
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* meantime new request added to hctx->dispatch is missed to check in
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* blk_mq_run_hw_queue().
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*/
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smp_mb();
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blk_mq_run_hw_queue(hctx, true);
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blk_mq_run_hw_queue(hctx, true);
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}
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}
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@ -1318,6 +1318,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
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list_splice_tail_init(list, &hctx->dispatch);
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list_splice_tail_init(list, &hctx->dispatch);
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spin_unlock(&hctx->lock);
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spin_unlock(&hctx->lock);
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/*
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* Order adding requests to hctx->dispatch and checking
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* SCHED_RESTART flag. The pair of this smp_mb() is the one
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* in blk_mq_sched_restart(). Avoid restart code path to
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* miss the new added requests to hctx->dispatch, meantime
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* SCHED_RESTART is observed here.
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*/
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smp_mb();
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/*
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/*
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* If SCHED_RESTART was set by the caller of this function and
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* If SCHED_RESTART was set by the caller of this function and
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* it is no longer set that means that it was cleared by another
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* it is no longer set that means that it was cleared by another
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