Re: [PATCH for-next] dm: fix missing bi_remaining accounting

[Mikulas Patocka]

On Fri, 1 Nov 2013, Jens Axboe wrote:

> On 11/01/2013 07:59 AM, Mike Snitzer wrote:
> > Add the missing bi_remaining increment, required by the block layer's
> > new bio-chaining code, to both the verity and old snapshot DM targets.
> > 
> > Otherwise users will hit the bi_remaining <= 0 BUG_ON in bio_endio().
> 
> Thanks Mike, added to the mix.
> 
> -- 
> Jens Axboe

Hi

This improves a little bit on the previous patch, by replacing costly 
atomic_inc with cheap atomic_set.


From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

dm: change atomic_inc to atomic_set(1)

There are places in dm where we save bi_endio and bi_private, set them to
target's routine, submit the bio, from the target's bi_endio routine we
restore bi_endio and bi_private and end the bio with bi_endio.

This causes underflow of bi_remaining, so we must restore bi_remaining
before ending the bio from the target bi_endio routine.

The code uses atomic_inc for restoration of bi_remaining. This patch
changes it to atomic_set(1) to avoid an interlocked instruction. In the
target's bi_endio routine we are sure that bi_remaining is zero
(otherwise, the bi_endio routine wouldn't be called) and there are no
concurrent users of the bio, so we can replace atomic_inc with
atomic_set(1).

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

---
 drivers/md/dm-cache-target.c |    2 +-
 drivers/md/dm-snap.c         |    2 +-
 drivers/md/dm-thin.c         |    4 ++--
 drivers/md/dm-verity.c       |    2 +-
 4 files changed, 5 insertions(+), 5 deletions(-)

Index: linux-block/drivers/md/dm-cache-target.c
===================================================================
--- linux-block.orig/drivers/md/dm-cache-target.c	2013-11-01 17:21:11.000000000 +0100
+++ linux-block/drivers/md/dm-cache-target.c	2013-11-04 13:59:13.000000000 +0100
@@ -670,7 +670,7 @@ static void writethrough_endio(struct bi
 	 * Must bump bi_remaining to allow bio to complete with
 	 * restored bi_end_io.
 	 */
-	atomic_inc(&bio->bi_remaining);
+	atomic_set(&bio->bi_remaining, 1);
 
 	if (err) {
 		bio_endio(bio, err);
Index: linux-block/drivers/md/dm-snap.c
===================================================================
--- linux-block.orig/drivers/md/dm-snap.c	2013-11-01 17:21:11.000000000 +0100
+++ linux-block/drivers/md/dm-snap.c	2013-11-04 13:59:56.000000000 +0100
@@ -1415,7 +1415,7 @@ out:
 	if (full_bio) {
 		full_bio->bi_end_io = pe->full_bio_end_io;
 		full_bio->bi_private = pe->full_bio_private;
-		atomic_inc(&full_bio->bi_remaining);
+		atomic_set(&full_bio->bi_remaining, 1);
 	}
 	free_pending_exception(pe);
 
Index: linux-block/drivers/md/dm-thin.c
===================================================================
--- linux-block.orig/drivers/md/dm-thin.c	2013-11-01 17:21:11.000000000 +0100
+++ linux-block/drivers/md/dm-thin.c	2013-11-04 14:00:19.000000000 +0100
@@ -613,7 +613,7 @@ static void process_prepared_mapping_fai
 {
 	if (m->bio) {
 		m->bio->bi_end_io = m->saved_bi_end_io;
-		atomic_inc(&m->bio->bi_remaining);
+		atomic_set(&m->bio->bi_remaining, 1);
 	}
 	cell_error(m->tc->pool, m->cell);
 	list_del(&m->list);
@@ -630,7 +630,7 @@ static void process_prepared_mapping(str
 	bio = m->bio;
 	if (bio) {
 		bio->bi_end_io = m->saved_bi_end_io;
-		atomic_inc(&bio->bi_remaining);
+		atomic_set(&bio->bi_remaining, 1);
 	}
 
 	if (m->err) {
Index: linux-block/drivers/md/dm-verity.c
===================================================================
--- linux-block.orig/drivers/md/dm-verity.c	2013-11-01 17:21:11.000000000 +0100
+++ linux-block/drivers/md/dm-verity.c	2013-11-04 13:59:28.000000000 +0100
@@ -384,7 +384,7 @@ static void verity_finish_io(struct dm_v
 
 	bio->bi_end_io = io->orig_bi_end_io;
 	bio->bi_private = io->orig_bi_private;
-	atomic_inc(&bio->bi_remaining);
+	atomic_set(&bio->bi_remaining, 1);
 
 	bio_endio(bio, error);
 }
--
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