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>
diff --git a/block/bio.c b/block/bio.c
index 117da31..c2ff8a8 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -271,7 +271,7 @@
memset(bio, 0, sizeof(*bio));
bio->bi_flags = 1 << BIO_UPTODATE;
atomic_set(&bio->__bi_remaining, 1);
- atomic_set(&bio->bi_cnt, 1);
+ atomic_set(&bio->__bi_cnt, 1);
}
EXPORT_SYMBOL(bio_init);
@@ -524,13 +524,17 @@
**/
void bio_put(struct bio *bio)
{
- BIO_BUG_ON(!atomic_read(&bio->bi_cnt));
-
- /*
- * last put frees it
- */
- if (atomic_dec_and_test(&bio->bi_cnt))
+ if (!bio_flagged(bio, BIO_REFFED))
bio_free(bio);
+ else {
+ BIO_BUG_ON(!atomic_read(&bio->__bi_cnt));
+
+ /*
+ * last put frees it
+ */
+ if (atomic_dec_and_test(&bio->__bi_cnt))
+ bio_free(bio);
+ }
}
EXPORT_SYMBOL(bio_put);