@@ -78,6 +78,15 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
return;
clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
+ /*
+ * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
+ * in blk_mq_run_hw_queue(). Its pair is the barrier in
+ * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
+ * meantime new request added to hctx->dispatch is missed to check in
+ * blk_mq_run_hw_queue().
+ */
+ smp_mb();
+
blk_mq_run_hw_queue(hctx, true);
}
@@ -1437,6 +1437,15 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
list_splice_tail_init(list, &hctx->dispatch);
spin_unlock(&hctx->lock);
+ /*
+ * Order adding requests to hctx->dispatch and checking
+ * SCHED_RESTART flag. The pair of this smp_mb() is the one
+ * in blk_mq_sched_restart(). Avoid restart code path to
+ * miss the new added requests to hctx->dispatch, meantime
+ * SCHED_RESTART is observed here.
+ */
+ smp_mb();
+
/*
* If SCHED_RESTART was set by the caller of this function and
* it is no longer set that means that it was cleared by another
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> Signed-off-by: Ming Lei <ming.lei@redhat.com> --- block/blk-mq-sched.c | 9 +++++++++ block/blk-mq.c | 9 +++++++++ 2 files changed, 18 insertions(+)