From 60b6a7e6a0f4382cd689f9afdac816964fec2921 Mon Sep 17 00:00:00 2001
From: Edward Hsieh <edwardh@synology.com>
Date: Thu, 24 Jun 2021 20:30:30 +0800
Subject: [PATCH] block: fix trace completion for chained bio
Git-commit: 60b6a7e6a0f4382cd689f9afdac816964fec2921
Patch-mainline: v5.14-rc1
References: bsc#1189505

For chained bio, trace_block_bio_complete in bio_endio is currently called
only by the parent bio once upon all chained bio completed.
However, the sector and size for the parent bio are modified in bio_split.
Therefore, the size and sector of the complete events might not match the
queue events in blktrace.

The original fix of bio completion trace <fbbaf700e7b1> ("block: trace
completion of all bios.") wants multiple complete events to correspond
to one queue event but missed this.

The issue can be reproduced by md/raid5 read with bio cross chunks.

To fix, move trace completion into the loop for every chained bio to call.

Fixes: fbbaf700e7b1 ("block: trace completion of all bios.")
Reviewed-by: Wade Liang <wadel@synology.com>
Reviewed-by: BingJing Chang <bingjingc@synology.com>
Signed-off-by: Edward Hsieh <edwardh@synology.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210624123030.27014-1-edwardh@synology.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Jan Kara <jack@suse.cz>

---
 block/bio.c |   15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

--- a/block/bio.c
+++ b/block/bio.c
@@ -1783,8 +1783,7 @@ static inline bool bio_remaining_done(st
  *
  *   bio_endio() can be called several times on a bio that has been chained
  *   using bio_chain().  The ->bi_end_io() function will only be called the
- *   last time.  At this point the BLK_TA_COMPLETE tracing event will be
- *   generated if BIO_TRACE_COMPLETION is set.
+ *   last time.
  **/
 void bio_endio(struct bio *bio)
 {
@@ -1794,6 +1793,12 @@ again:
 	if (!bio_integrity_endio(bio))
 		return;
 
+	if (bio->bi_disk && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
+		trace_block_bio_complete(bio->bi_disk->queue, bio,
+					 blk_status_to_errno(bio->bi_status));
+		bio_clear_flag(bio, BIO_TRACE_COMPLETION);
+	}
+
 	/*
 	 * Need to have a real endio function for chained bios, otherwise
 	 * various corner cases will break (like stacking block devices that
@@ -1807,12 +1812,6 @@ again:
 		goto again;
 	}
 
-	if (bio->bi_disk && bio_flagged(bio, BIO_TRACE_COMPLETION)) {
-		trace_block_bio_complete(bio->bi_disk->queue, bio,
-					 blk_status_to_errno(bio->bi_status));
-		bio_clear_flag(bio, BIO_TRACE_COMPLETION);
-	}
-
 	blk_throtl_bio_endio(bio);
 	/* release cgroup info */
 	bio_uninit(bio);