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authorMuchun Song <songmuchun@bytedance.com>2024-10-14 17:29:34 +0800
committerJens Axboe <axboe@kernel.dk>2024-10-22 08:16:40 -0600
commit96a9fe64bfd486ebeeacf1e6011801ffe89dae18 (patch)
tree4df46f7869f74a39c3e90a72d9a464e90bfd3d87 /block
parent6bda857bcbb86fb9d0e54fbef93a093d51172acc (diff)
block: fix ordering between checking BLK_MQ_S_STOPPED request adding
Supposing first scenario with a virtio_blk driver. CPU0 CPU1 blk_mq_try_issue_directly() __blk_mq_issue_directly() q->mq_ops->queue_rq() virtio_queue_rq() blk_mq_stop_hw_queue() virtblk_done() blk_mq_request_bypass_insert() 1) store blk_mq_start_stopped_hw_queue() clear_bit(BLK_MQ_S_STOPPED) 3) store blk_mq_run_hw_queue() if (!blk_mq_hctx_has_pending()) 4) load return blk_mq_sched_dispatch_requests() blk_mq_run_hw_queue() if (!blk_mq_hctx_has_pending()) return blk_mq_sched_dispatch_requests() if (blk_mq_hctx_stopped()) 2) load return __blk_mq_sched_dispatch_requests() Supposing another scenario. CPU0 CPU1 blk_mq_requeue_work() blk_mq_insert_request() 1) store virtblk_done() blk_mq_start_stopped_hw_queue() blk_mq_run_hw_queues() clear_bit(BLK_MQ_S_STOPPED) 3) store blk_mq_run_hw_queue() if (!blk_mq_hctx_has_pending()) 4) load return blk_mq_sched_dispatch_requests() if (blk_mq_hctx_stopped()) 2) load continue blk_mq_run_hw_queue() Both scenarios are similar, the full memory barrier should be inserted between 1) and 2), as well as between 3) and 4) to make sure that either CPU0 sees BLK_MQ_S_STOPPED is cleared or CPU1 sees dispatch list. Otherwise, either CPU will not rerun the hardware queue causing starvation of the request. The easy way to fix it is to add the essential full memory barrier into helper of blk_mq_hctx_stopped(). In order to not affect the fast path (hardware queue is not stopped most of the time), we only insert the barrier into the slow path. Actually, only slow path needs to care about missing of dispatching the request to the low-level device driver. Fixes: 320ae51feed5 ("blk-mq: new multi-queue block IO queueing mechanism") Cc: stable@vger.kernel.org Cc: Muchun Song <muchun.song@linux.dev> Signed-off-by: Muchun Song <songmuchun@bytedance.com> Reviewed-by: Ming Lei <ming.lei@redhat.com> Link: https://lore.kernel.org/r/20241014092934.53630-4-songmuchun@bytedance.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block')
-rw-r--r--block/blk-mq.c6
-rw-r--r--block/blk-mq.h13
2 files changed, 19 insertions, 0 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c
index bb4ee2380dce..022653320c18 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2438,6 +2438,12 @@ void blk_mq_start_stopped_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
return;
clear_bit(BLK_MQ_S_STOPPED, &hctx->state);
+ /*
+ * Pairs with the smp_mb() in blk_mq_hctx_stopped() to order the
+ * clearing of BLK_MQ_S_STOPPED above and the checking of dispatch
+ * list in the subsequent routine.
+ */
+ smp_mb__after_atomic();
blk_mq_run_hw_queue(hctx, async);
}
EXPORT_SYMBOL_GPL(blk_mq_start_stopped_hw_queue);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 3bd43b10032f..f4ac1af77a26 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -230,6 +230,19 @@ static inline struct blk_mq_tags *blk_mq_tags_from_data(struct blk_mq_alloc_data
static inline bool blk_mq_hctx_stopped(struct blk_mq_hw_ctx *hctx)
{
+ /* Fast path: hardware queue is not stopped most of the time. */
+ if (likely(!test_bit(BLK_MQ_S_STOPPED, &hctx->state)))
+ return false;
+
+ /*
+ * This barrier is used to order adding of dispatch list before and
+ * the test of BLK_MQ_S_STOPPED below. Pairs with the memory barrier
+ * in blk_mq_start_stopped_hw_queue() so that dispatch code could
+ * either see BLK_MQ_S_STOPPED is cleared or dispatch list is not
+ * empty to avoid missing dispatching requests.
+ */
+ smp_mb();
+
return test_bit(BLK_MQ_S_STOPPED, &hctx->state);
}