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authorMing Lei <ming.lei@redhat.com>2020-08-17 18:01:15 +0800
committerJens Axboe <axboe@kernel.dk>2020-08-17 06:57:49 -0700
commitd7d8535f377e9ba87edbf7fbbd634ac942f3f54f (patch)
tree22e6b5a07c194baa82aa448ff5f1fd414cf7456c /block/blk-mq-sched.c
parent03ef5941a04c68b5eb8254493d09b6e899a377a3 (diff)
downloadlwn-d7d8535f377e9ba87edbf7fbbd634ac942f3f54f.tar.gz
lwn-d7d8535f377e9ba87edbf7fbbd634ac942f3f54f.zip
blk-mq: order adding requests to hctx->dispatch and checking SCHED_RESTART
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: bd166ef183c2 ("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>
Diffstat (limited to 'block/blk-mq-sched.c')
-rw-r--r--block/blk-mq-sched.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index a19cdf159b75..d2790e5b06d1 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -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);
}