[PATCH 5.4 152/214] blk-mq: order adding requests to hctx->dispatch and checking SCHED_RESTART

Greg Kroah-Hartman
2020-09-01T15:33:07Z

From: Ming Lei <ming.lei@redhat.com>

commit d7d8535f377e9ba87edbf7fbbd634ac942f3f54f 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: 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
block/blk-mq-sched.c | 9 +++++++++
block/blk-mq.c | 9 +++++++++
2 files changed, 18 insertions(+)

--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -77,6 +77,15 @@ void blk_mq_sched_restart(struct blk_mq_
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);
}

--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1319,6 +1319,15 @@ bool blk_mq_dispatch_rq_list(struct requ
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
* thread and hence that a queue rerun is needed.