From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Bart Van Assche To: Jens Axboe Cc: linux-block@vger.kernel.org, Christoph Hellwig , Bart Van Assche , Ming Lei , Omar Sandoval , Hannes Reinecke , Johannes Thumshirn Subject: [PATCH 7/7] blk-mq: Fix another queue stall Date: Thu, 30 Nov 2017 16:08:48 -0800 Message-Id: <20171201000848.2656-8-bart.vanassche@wdc.com> In-Reply-To: <20171201000848.2656-1-bart.vanassche@wdc.com> References: <20171201000848.2656-1-bart.vanassche@wdc.com> List-ID: The following code at the end of blk_mq_dispatch_rq_list() detects whether or not wake_up(&hctx->dispatch_wait) has been called concurrently with pushing back requests onto the dispatch list: list_empty_careful(&hctx->dispatch_wait.entry) Since blk_mq_dispatch_wake() is protected by another lock than the dispatch list and since blk_mq_run_hw_queue() does not acquire any lock if it notices that no requests are pending, blk_mq_dispatch_wake() is not ordered against the code that pushes back requests onto the dispatch list. Avoid that the dispatch_wait empty check fails due to load/store reordering by serializing it against the dispatch_wait queue wakeup. This patch fixes a queue stall I ran into while testing a SCSI initiator driver with the maximum target depth set to one. Signed-off-by: Bart Van Assche Cc: Ming Lei Cc: Omar Sandoval Cc: Christoph Hellwig Cc: Hannes Reinecke Cc: Johannes Thumshirn --- block/blk-mq.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index b4225f606737..a11767a4d95c 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1074,6 +1074,20 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx, return ret; } +static bool blk_mq_dispatch_list_empty(struct blk_mq_hw_ctx *hctx) +{ + struct sbq_wait_state *ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx); + struct wait_queue_head *wq_head = &ws->wait; + unsigned long flags; + bool result; + + spin_lock_irqsave(&wq_head->lock, flags); + result = list_empty(&hctx->dispatch_wait.entry); + spin_unlock_irqrestore(&wq_head->lock, flags); + + return result; +} + bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, bool got_budget) { @@ -1197,7 +1211,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, */ if (restart || !blk_mq_sched_needs_restart(hctx) || - (no_tag && list_empty_careful(&hctx->dispatch_wait.entry))) + (no_tag && blk_mq_dispatch_list_empty(hctx))) blk_mq_run_hw_queue(hctx, true); } -- 2.15.0