2 files changed, 37 insertions, 0 deletions

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 1784e50eb857..3be0fcc19b1f 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -330,6 +330,14 @@ struct cached_dev {
 
 	struct keybuf		writeback_keys;
 
+	/*
+	 * Order the write-half of writeback operations strongly in dispatch
+	 * order.  (Maintain LBA order; don't allow reads completing out of
+	 * order to re-order the writes...)
+	 */
+	struct closure_waitlist writeback_ordering_wait;
+	atomic_t		writeback_sequence_next;
+
 	/* For tracking sequential IO */
 #define RECENT_IO_BITS	7
 #define RECENT_IO	(1 << RECENT_IO_BITS)
diff --git a/drivers/md/bcache/writeback.c b/drivers/md/bcache/writeback.c
index 479095987f22..6e1d2fde43df 100644
--- a/drivers/md/bcache/writeback.c
+++ b/drivers/md/bcache/writeback.c
@@ -116,6 +116,7 @@ static unsigned writeback_delay(struct cached_dev *dc, unsigned sectors)
 struct dirty_io {
 	struct closure		cl;
 	struct cached_dev	*dc;
+	uint16_t		sequence;
 	struct bio		bio;
 };
 
@@ -194,6 +195,27 @@ static void write_dirty(struct closure *cl)
 {
 	struct dirty_io *io = container_of(cl, struct dirty_io, cl);
 	struct keybuf_key *w = io->bio.bi_private;
+	struct cached_dev *dc = io->dc;
+
+	uint16_t next_sequence;
+
+	if (atomic_read(&dc->writeback_sequence_next) != io->sequence) {
+		/* Not our turn to write; wait for a write to complete */
+		closure_wait(&dc->writeback_ordering_wait, cl);
+
+		if (atomic_read(&dc->writeback_sequence_next) == io->sequence) {
+			/*
+			 * Edge case-- it happened in indeterminate order
+			 * relative to when we were added to wait list..
+			 */
+			closure_wake_up(&dc->writeback_ordering_wait);
+		}
+
+		continue_at(cl, write_dirty, io->dc->writeback_write_wq);
+		return;
+	}
+
+	next_sequence = io->sequence + 1;
 
 	/*
 	 * IO errors are signalled using the dirty bit on the key.
@@ -211,6 +233,9 @@ static void write_dirty(struct closure *cl)
 		closure_bio_submit(&io->bio, cl);
 	}
 
+	atomic_set(&dc->writeback_sequence_next, next_sequence);
+	closure_wake_up(&dc->writeback_ordering_wait);
+
 	continue_at(cl, write_dirty_finish, io->dc->writeback_write_wq);
 }
 
@@ -242,7 +267,10 @@ static void read_dirty(struct cached_dev *dc)
 	int nk, i;
 	struct dirty_io *io;
 	struct closure cl;
+	uint16_t sequence = 0;
 
+	BUG_ON(!llist_empty(&dc->writeback_ordering_wait.list));
+	atomic_set(&dc->writeback_sequence_next, sequence);
 	closure_init_stack(&cl);
 
 	/*
@@ -303,6 +331,7 @@ static void read_dirty(struct cached_dev *dc)
 
 			w->private	= io;
 			io->dc		= dc;
+			io->sequence    = sequence++;
 
 			dirty_init(w);
 			bio_set_op_attrs(&io->bio, REQ_OP_READ, 0);