linux-kernel - [PATCH] block: fix false warning in bdev_count_inflight

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Message-Id: <20250626083927.576207-1-yukuai1@huaweicloud.com>
Date: Thu, 26 Jun 2025 16:39:27 +0800
From: Yu Kuai <yukuai1@...weicloud.com>
To: axboe@...nel.dk,
	hare@...e.de,
	hch@...radead.org,
	yukuai3@...wei.com,
	john.g.garry@...cle.com
Cc: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	yi.zhang@...hat.com,
	calvin@...nvd.org,
	david@...morbit.com,
	yukuai1@...weicloud.com,
	yi.zhang@...wei.com,
	yangerkun@...wei.com,
	johnny.chenyi@...wei.com
Subject: [PATCH] block: fix false warning in bdev_count_inflight_rw()

From: Yu Kuai <yukuai3@...wei.com>

While bdev_count_inflight is interating all cpus, if some IOs are issued
from traversed cpu and then completed from the cpu that is not traversed
yet:

cpu0
		cpu1
		bdev_count_inflight
		 //for_each_possible_cpu
		 // cpu0 is 0
		 infliht += 0
// issue a io
blk_account_io_start
// cpu0 inflight ++

				cpu2
				// the io is done
				blk_account_io_done
				// cpu2 inflight --
		 // cpu 1 is 0
		 inflight += 0
		 // cpu2 is -1
		 inflight += -1
		 ...

In this case, the total inflight will be -1, causing lots of false
warning. Fix the problem by removing the warning.

Noted there is still a valid warning for nvme-mpath(From Yi) that is not
fixed yet.

Fixes: f5482ee5edb9 ("block: WARN if bdev inflight counter is negative")
Reported-by: Yi Zhang <yi.zhang@...hat.com>
Closes: https://lore.kernel.org/linux-block/aFtUXy-lct0WxY2w@mozart.vkv.me/T/#mae89155a5006463d0a21a4a2c35ae0034b26a339
Reported-and-tested-by: Calvin Owens <calvin@...nvd.org>
Closes: https://lore.kernel.org/linux-block/aFtUXy-lct0WxY2w@mozart.vkv.me/T/#m1d935a00070bf95055d0ac84e6075158b08acaef
Reported-by: Dave Chinner <david@...morbit.com>
Closes: https://lore.kernel.org/linux-block/aFuypjqCXo9-5_En@dread.disaster.area/
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
 block/genhd.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 8171a6bc3210..680fa717082f 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -141,9 +141,14 @@ static void bdev_count_inflight_rw(struct block_device *part,
 		}
 	}
 
-	if (WARN_ON_ONCE((int)inflight[READ] < 0))
+	/*
+	 * While iterating all cpus, some IOs might issued from traversed cpu
+	 * and then completed from the cpu that is not traversed yet, causing
+	 * the inflight number to be negative.
+	 */
+	if ((int)inflight[READ] < 0)
 		inflight[READ] = 0;
-	if (WARN_ON_ONCE((int)inflight[WRITE] < 0))
+	if ((int)inflight[WRITE] < 0)
 		inflight[WRITE] = 0;
 }
 
-- 
2.39.2