block: use singly linked list for bio cache

Pointless to maintain a head/tail for the list, as we never need to
access the tail. Entries are always LIFO for cache hotness reasons.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -26,7 +26,7 @@
 #include "blk-rq-qos.h"
 
 struct bio_alloc_cache {
-	struct bio_list		free_list;
+	struct bio		*free_list;
 	unsigned int		nr;
 };
 
@@ -630,7 +630,8 @@ static void bio_alloc_cache_prune(struct bio_alloc_cache *cache,
 	unsigned int i = 0;
 	struct bio *bio;
 
-	while ((bio = bio_list_pop(&cache->free_list)) != NULL) {
+	while ((bio = cache->free_list) != NULL) {
+		cache->free_list = bio->bi_next;
 		cache->nr--;
 		bio_free(bio);
 		if (++i == nr)
@@ -689,7 +690,8 @@ void bio_put(struct bio *bio)
 
 		bio_uninit(bio);
 		cache = per_cpu_ptr(bio->bi_pool->cache, get_cpu());
-		bio_list_add_head(&cache->free_list, bio);
+		bio->bi_next = cache->free_list;
+		cache->free_list = bio;
 		if (++cache->nr > ALLOC_CACHE_MAX + ALLOC_CACHE_SLACK)
 			bio_alloc_cache_prune(cache, ALLOC_CACHE_SLACK);
 		put_cpu();
@@ -1704,8 +1706,9 @@ struct bio *bio_alloc_kiocb(struct kiocb *kiocb, unsigned short nr_vecs,
 		return bio_alloc_bioset(GFP_KERNEL, nr_vecs, bs);
 
 	cache = per_cpu_ptr(bs->cache, get_cpu());
-	bio = bio_list_pop(&cache->free_list);
-	if (bio) {
+	if (cache->free_list) {
+		bio = cache->free_list;
+		cache->free_list = bio->bi_next;
 		cache->nr--;
 		put_cpu();
 		bio_init(bio, nr_vecs ? bio->bi_inline_vecs : NULL, nr_vecs);