From: Li Nan <redacted>

/sys/block/[device]/queue/iostats is used to control whether to count io
stat. Write 0 to it will clear queue_flags QUEUE_FLAG_IO_STAT which means
iostats is disabled. If we disable iostats and later endable it, the io
issued during this period will be counted incorrectly, inflight will be
decreased to -1.

   //T1 set iostats
   echo 0 > /sys/block/md0/queue/iostats
    clear QUEUE_FLAG_IO_STAT

			//T2 issue io
			if (QUEUE_FLAG_IO_STAT) -> false
			 bio_start_io_acct
			  inflight++

   echo 1 > /sys/block/md0/queue/iostats
    set QUEUE_FLAG_IO_STAT

					//T3 io end
					if (QUEUE_FLAG_IO_STAT) -> true
					 bio_end_io_acct
					  inflight--	-> -1

Also, if iostats is enabled while issuing io but disabled while io end,
inflight will never be decreased.

Fix it by checking start_time when io end. Only check QUEUE_FLAG_IO_STAT
while issuing io, just like request based devices.

Fixes: 528bc2cf2fcc ("md/raid10: enable io accounting")
Signed-off-by: Li Nan <redacted>
---
  drivers/md/raid10.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 381c21f7fb06..bf9dca5c25c3 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c

@@ -325,7 +325,7 @@ static void raid_end_bio_io(struct r10bio *r10_bio)
  	if (!test_bit(R10BIO_Uptodate, &r10_bio->state))
  		bio->bi_status = BLK_STS_IOERR;
  
-	if (blk_queue_io_stat(bio->bi_bdev->bd_disk->queue))
+	if (r10_bio->start_time)
  		bio_end_io_acct(bio, r10_bio->start_time);