block: Generic bio chaining

This adds a generic mechanism for chaining bio completions. This is
going to be used for a bio_split() replacement, and it turns out to be
very useful in a fair amount of driver code - a fair number of drivers
were implementing this in their own roundabout ways, often painfully.

Note that this means it's no longer to call bio_endio() more than once
on the same bio! This can cause problems for drivers that save/restore
bi_end_io. Arguably they shouldn't be saving/restoring bi_end_io at all
- in all but the simplest cases they'd be better off just cloning the
bio, and immutable biovecs is making bio cloning cheaper. But for now,
we add a bio_endio_nodec() for these cases.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>

9 files changed, 90 insertions, 11 deletions

diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
index 0f0ab659914d..522f95778443 100644
--- a/drivers/md/bcache/io.c
+++ b/drivers/md/bcache/io.c
@@ -133,7 +133,7 @@ static void bch_bio_submit_split_done(struct closure *cl)
 
         s->bio->bi_end_io = s->bi_end_io;
         s->bio->bi_private = s->bi_private;
-        bio_endio(s->bio, 0);
+        bio_endio_nodec(s->bio, 0);
 
         closure_debug_destroy(&s->cl);
         mempool_free(s, s->p->bio_split_hook);
diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 86f9c83eb30c..bf3a206abd78 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -765,6 +765,12 @@ static void writethrough_endio(struct bio *bio, int err)
 
         dm_unhook_bio(&pb->hook_info, bio);
 
+        /*
+         * Must bump bi_remaining to allow bio to complete with
+         * restored bi_end_io.
+         */
+        atomic_inc(&bio->bi_remaining);
+
         if (err) {
                 bio_endio(bio, err);
                 return;
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 3ded8c729dfb..80b5cabbea29 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1415,6 +1415,7 @@ out:
         if (full_bio) {
                 full_bio->bi_end_io = pe->full_bio_end_io;
                 full_bio->bi_private = pe->full_bio_private;
+                atomic_inc(&full_bio->bi_remaining);
         }
         free_pending_exception(pe);
 
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index a65402480c8c..1abb4a24c338 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -611,8 +611,10 @@ static void cell_defer_no_holder(struct thin_c *tc, struct dm_bio_prison_cell *c
 
 static void process_prepared_mapping_fail(struct dm_thin_new_mapping *m)
 {
-        if (m->bio)
+        if (m->bio) {
                 m->bio->bi_end_io = m->saved_bi_end_io;
+                atomic_inc(&m->bio->bi_remaining);
+        }
         cell_error(m->tc->pool, m->cell);
         list_del(&m->list);
         mempool_free(m, m->tc->pool->mapping_pool);
@@ -626,8 +628,10 @@ static void process_prepared_mapping(struct dm_thin_new_mapping *m)
         int r;
 
         bio = m->bio;
-        if (bio)
+        if (bio) {
                 bio->bi_end_io = m->saved_bi_end_io;
+                atomic_inc(&bio->bi_remaining);
+        }
 
         if (m->err) {
                 cell_error(pool, m->cell);
diff --git a/drivers/md/dm-verity.c b/drivers/md/dm-verity.c
index ac35e959d49b..796007a5e0e1 100644
--- a/drivers/md/dm-verity.c
+++ b/drivers/md/dm-verity.c
@@ -385,7 +385,7 @@ static void verity_finish_io(struct dm_verity_io *io, int error)
         bio->bi_end_io = io->orig_bi_end_io;
         bio->bi_private = io->orig_bi_private;
 
-        bio_endio(bio, error);
+        bio_endio_nodec(bio, error);
 }
 
 static void verity_work(struct work_struct *w)
diff --git a/fs/bio-integrity.c b/fs/bio-integrity.c
index fed744b8c9e5..9d547d2e357c 100644
--- a/fs/bio-integrity.c
+++ b/fs/bio-integrity.c
@@ -502,7 +502,7 @@ static void bio_integrity_verify_fn(struct work_struct *work)
 
         /* Restore original bio completion handler */
         bio->bi_end_io = bip->bip_end_io;
-        bio_endio(bio, error);
+        bio_endio_nodec(bio, error);
 }
 
 /**
diff --git a/fs/bio.c b/fs/bio.c
index e6dfa06773ac..b0a16dbc71ef 100644
--- a/fs/bio.c
+++ b/fs/bio.c
@@ -273,6 +273,7 @@ void bio_init(struct bio *bio)
 {
         memset(bio, 0, sizeof(*bio));
         bio->bi_flags = 1 << BIO_UPTODATE;
+        atomic_set(&bio->bi_remaining, 1);
         atomic_set(&bio->bi_cnt, 1);
 }
 EXPORT_SYMBOL(bio_init);
@@ -295,9 +296,35 @@ void bio_reset(struct bio *bio)
 
         memset(bio, 0, BIO_RESET_BYTES);
         bio->bi_flags = flags|(1 << BIO_UPTODATE);
+        atomic_set(&bio->bi_remaining, 1);
 }
 EXPORT_SYMBOL(bio_reset);
 
+static void bio_chain_endio(struct bio *bio, int error)
+{
+        bio_endio(bio->bi_private, error);
+        bio_put(bio);
+}
+
+/**
+ * bio_chain - chain bio completions
+ *
+ * The caller won't have a bi_end_io called when @bio completes - instead,
+ * @parent's bi_end_io won't be called until both @parent and @bio have
+ * completed; the chained bio will also be freed when it completes.
+ *
+ * The caller must not set bi_private or bi_end_io in @bio.
+ */
+void bio_chain(struct bio *bio, struct bio *parent)
+{
+        BUG_ON(bio->bi_private || bio->bi_end_io);
+
+        bio->bi_private = parent;
+        bio->bi_end_io  = bio_chain_endio;
+        atomic_inc(&parent->bi_remaining);
+}
+EXPORT_SYMBOL(bio_chain);
+
 static void bio_alloc_rescue(struct work_struct *work)
 {
         struct bio_set *bs = container_of(work, struct bio_set, rescue_work);
@@ -1719,16 +1746,53 @@ EXPORT_SYMBOL(bio_flush_dcache_pages);
  **/
 void bio_endio(struct bio *bio, int error)
 {
-        if (error)
-                clear_bit(BIO_UPTODATE, &bio->bi_flags);
-        else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
-                error = -EIO;
+        while (bio) {
+                BUG_ON(atomic_read(&bio->bi_remaining) <= 0);
+
+                if (error)
+                        clear_bit(BIO_UPTODATE, &bio->bi_flags);
+                else if (!test_bit(BIO_UPTODATE, &bio->bi_flags))
+                        error = -EIO;
+
+                if (!atomic_dec_and_test(&bio->bi_remaining))
+                        return;
 
-        if (bio->bi_end_io)
-                bio->bi_end_io(bio, error);
+                /*
+                 * Need to have a real endio function for chained bios,
+                 * otherwise various corner cases will break (like stacking
+                 * block devices that save/restore bi_end_io) - however, we want
+                 * to avoid unbounded recursion and blowing the stack. Tail call
+                 * optimization would handle this, but compiling with frame
+                 * pointers also disables gcc's sibling call optimization.
+                 */
+                if (bio->bi_end_io == bio_chain_endio) {
+                        struct bio *parent = bio->bi_private;
+                        bio_put(bio);
+                        bio = parent;
+                } else {
+                        if (bio->bi_end_io)
+                                bio->bi_end_io(bio, error);
+                        bio = NULL;
+                }
+        }
 }
 EXPORT_SYMBOL(bio_endio);
 
+/**
+ * bio_endio_nodec - end I/O on a bio, without decrementing bi_remaining
+ * @bio:        bio
+ * @error:      error, if any
+ *
+ * For code that has saved and restored bi_end_io; thing hard before using this
+ * function, probably you should've cloned the entire bio.
+ **/
+void bio_endio_nodec(struct bio *bio, int error)
+{
+        atomic_inc(&bio->bi_remaining);
+        bio_endio(bio, error);
+}
+EXPORT_SYMBOL(bio_endio_nodec);
+
 void bio_pair_release(struct bio_pair *bp)
 {
         if (atomic_dec_and_test(&bp->cnt)) {
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 0c32a45a419c..64f5169c224b 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -356,6 +356,7 @@ static inline struct bio *bio_clone_kmalloc(struct bio *bio, gfp_t gfp_mask)
 }
 
 extern void bio_endio(struct bio *, int);
+extern void bio_endio_nodec(struct bio *, int);
 struct request_queue;
 extern int bio_phys_segments(struct request_queue *, struct bio *);
 
@@ -364,6 +365,7 @@ extern void bio_advance(struct bio *, unsigned);
 
 extern void bio_init(struct bio *);
 extern void bio_reset(struct bio *);
+void bio_chain(struct bio *, struct bio *);
 
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index d369f8f6af79..bbc3a6c88fce 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -65,6 +65,8 @@ struct bio {
         unsigned int            bi_seg_front_size;
         unsigned int            bi_seg_back_size;
 
+        atomic_t                bi_remaining;
+
         bio_end_io_t            *bi_end_io;
 
         void                    *bi_private;