block: fix false warning in bdev_count_inflight_rw()

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@redhat.com>
Closes: https://lore.kernel.org/linux-block/aFtUXy-lct0WxY2w@mozart.vkv.me/T/#mae89155a5006463d0a21a4a2c35ae0034b26a339
Reported-and-tested-by: Calvin Owens <calvin@wbinvd.org>
Closes: https://lore.kernel.org/linux-block/aFtUXy-lct0WxY2w@mozart.vkv.me/T/#m1d935a00070bf95055d0ac84e6075158b08acaef
Reported-by: Dave Chinner <david@fromorbit.com>
Closes: https://lore.kernel.org/linux-block/aFuypjqCXo9-5_En@dread.disaster.area/
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250626115743.1641443-1-yukuai3@huawei.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/genhd.c b/block/genhd.c
index 8171a6bc3210fc8e9990707994ba4e38c816529a..c26733f6324b250be7510631341478ac61c42214 100644 (file)
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -128,23 +128,27 @@ static void part_stat_read_all(struct block_device *part,
 static void bdev_count_inflight_rw(struct block_device *part,
                unsigned int inflight[2], bool mq_driver)
 {
+       int write = 0;
+       int read = 0;
        int cpu;
 
        if (mq_driver) {
                blk_mq_in_driver_rw(part, inflight);
-       } else {
-               for_each_possible_cpu(cpu) {
-                       inflight[READ] += part_stat_local_read_cpu(
-                                               part, in_flight[READ], cpu);
-                       inflight[WRITE] += part_stat_local_read_cpu(
-                                               part, in_flight[WRITE], cpu);
-               }
+               return;
+       }
+
+       for_each_possible_cpu(cpu) {
+               read += part_stat_local_read_cpu(part, in_flight[READ], cpu);
+               write += part_stat_local_read_cpu(part, in_flight[WRITE], cpu);
        }
 
-       if (WARN_ON_ONCE((int)inflight[READ] < 0))
-               inflight[READ] = 0;
-       if (WARN_ON_ONCE((int)inflight[WRITE] < 0))
-               inflight[WRITE] = 0;
+       /*
+        * While iterating all CPUs, some IOs may be issued from a CPU already
+        * traversed and complete on a CPU that has not yet been traversed,
+        * causing the inflight number to be negative.
+        */
+       inflight[READ] = read > 0 ? read : 0;
+       inflight[WRITE] = write > 0 ? write : 0;
 }
 
 /**