block: make bio_inc_remaining() interface accessible again

Commit 326e1dbb57 ("block: remove management of bi_remaining when
restoring original bi_end_io") made bio_inc_remaining() private to bio.c
because the only use-case that made sense was confined to the
bio_chain() interface.

Since that time DM thinp went on to use bio_chain() in its relatively
complex implementation of async discard support.  That implementation,
even when converted over to use the new async __blkdev_issue_discard()
interface, depends on deferred completion of the original discard bio --
which is most appropriately implemented using bio_inc_remaining().

DM thinp foolishly duplicated bio_inc_remaining(), local to dm-thin.c as
__bio_inc_remaining(), so re-exporting bio_inc_remaining() allows us to
put an end to that foolishness.

All said, bio_inc_remaining() should really only be used in conjunction
with bio_chain().  It isn't intended for generic bio reference counting.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 6b7481f62218..9faebf7f9a33 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -703,6 +703,17 @@ static inline struct bio *bio_list_get(struct bio_list *bl)
 }
 
 /*
+ * Increment chain count for the bio. Make sure the CHAIN flag update
+ * is visible before the raised count.
+ */
+static inline void bio_inc_remaining(struct bio *bio)
+{
+	bio_set_flag(bio, BIO_CHAIN);
+	smp_mb__before_atomic();
+	atomic_inc(&bio->__bi_remaining);
+}
+
+/*
  * bio_set is used to allow other portions of the IO system to
  * allocate their own private memory pools for bio and iovec structures.
  * These memory pools in turn all allocate from the bio_slab