From: Joe Thornber <ejt@redhat.com>
Subject: dm cache: add utility functions for hooking a bio's endio function

[FIXME: not convinced these belong in DM, could/should be elevated
 to block layer.  No benefit in dm-cache that I can see yet since it is
 only hooking/unhooking once]

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-cache-target.c |   35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

Index: linux/drivers/md/dm-cache-target.c
===================================================================
--- linux.orig/drivers/md/dm-cache-target.c
+++ linux/drivers/md/dm-cache-target.c
@@ -61,6 +61,34 @@ static void free_bitset(unsigned long *b
 
 /*----------------------------------------------------------------*/
 
+/*
+ * There are a couple of places where we let a bio run, but want to do some
+ * work before calling its endio function.  We do this by temporarily
+ * changing the endio fn.
+ */
+struct hook_info {
+	bio_end_io_t *bi_end_io;
+	void *bi_private;
+};
+
+static void hook_bio(struct hook_info *h, struct bio *bio,
+		     bio_end_io_t *bi_end_io, void *bi_private)
+{
+	h->bi_end_io = bio->bi_end_io;
+	h->bi_private = bio->bi_private;
+
+	bio->bi_end_io = bi_end_io;
+	bio->bi_private = bi_private;
+}
+
+static void unhook_bio(struct hook_info *h, struct bio *bio)
+{
+	bio->bi_end_io = h->bi_end_io;
+	bio->bi_private = h->bi_private;
+}
+
+/*----------------------------------------------------------------*/
+
 #define PRISON_CELLS 1024
 #define MIGRATION_POOL_SIZE 128
 #define COMMIT_PERIOD HZ
@@ -211,7 +239,7 @@ struct per_bio_data {
 	 */
 	struct cache *cache;
 	dm_cblock_t cblock;
-	bio_end_io_t *saved_bi_end_io;
+	struct hook_info hook_info;
 	struct dm_bio_details bio_details;
 };
 
@@ -662,7 +690,7 @@ static void defer_writethrough_bio(struc
 static void writethrough_endio(struct bio *bio, int err)
 {
 	struct per_bio_data *pb = get_per_bio_data(bio, PB_DATA_SIZE_WT);
-	bio->bi_end_io = pb->saved_bi_end_io;
+	unhook_bio(&pb->hook_info, bio);
 
 	if (err) {
 		bio_endio(bio, err);
@@ -693,9 +721,8 @@ static void remap_to_origin_then_cache(s
 
 	pb->cache = cache;
 	pb->cblock = cblock;
-	pb->saved_bi_end_io = bio->bi_end_io;
+	hook_bio(&pb->hook_info, bio, writethrough_endio, NULL);
 	dm_bio_record(&pb->bio_details, bio);
-	bio->bi_end_io = writethrough_endio;
 
 	remap_to_origin_clear_discard(pb->cache, bio, oblock);
 }