bio: skip atomic inc/dec of ->bi_cnt for most use cases

Struct bio has a reference count that controls when it can be freed.
Most uses cases is allocating the bio, which then returns with a
single reference to it, doing IO, and then dropping that single
reference. We can remove this atomic_dec_and_test() in the completion
path, if nobody else is holding a reference to the bio.

If someone does call bio_get() on the bio, then we flag the bio as
now having valid count and that we must properly honor the reference
count when it's being put.

Tested-by: Robert Elliott <elliott@hp.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 8bfe9eee6d1a..7486ea103f6e 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -290,7 +290,21 @@ static inline unsigned bio_segments(struct bio *bio)
  * returns. and then bio would be freed memory when if (bio->bi_flags ...)
  * runs
  */
-#define bio_get(bio)	atomic_inc(&(bio)->bi_cnt)
+static inline void bio_get(struct bio *bio)
+{
+	bio->bi_flags |= (1 << BIO_REFFED);
+	smp_mb__before_atomic();
+	atomic_inc(&bio->__bi_cnt);
+}
+
+static inline void bio_cnt_set(struct bio *bio, unsigned int count)
+{
+	if (count != 1) {
+		bio->bi_flags |= (1 << BIO_REFFED);
+		smp_mb__before_atomic();
+	}
+	atomic_set(&bio->__bi_cnt, count);
+}
 
 enum bip_flags {
 	BIP_BLOCK_INTEGRITY	= 1 << 0, /* block layer owns integrity data */