bcache: btree locking rework

Add a new lock, b->write_lock, which is required to actually modify - or write -
a btree node; this lock is only held for short durations.

This means we can write out a btree node without taking b->lock, which _is_ held
for long durations - solving a deadlock when btree_flush_write() (from the
journalling code) is called with a btree node locked.

Right now just occurs in bch_btree_set_root(), but with an upcoming journalling
rework is going to happen a lot more.

This also turns b->lock is now more of a read/intent lock instead of a
read/write lock - but not completely, since it still blocks readers. May turn it
into a real intent lock at some point in the future.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/drivers/md/bcache/btree.h b/drivers/md/bcache/btree.h
index def9dc4a822f..acebf26809cc 100644
--- a/drivers/md/bcache/btree.h
+++ b/drivers/md/bcache/btree.h
@@ -127,6 +127,8 @@ struct btree {
 	struct cache_set	*c;
 	struct btree		*parent;
 
+	struct mutex		write_lock;
+
 	unsigned long		flags;
 	uint16_t		written;	/* would be nice to kill */
 	uint8_t			level;
@@ -236,6 +238,7 @@ static inline void rw_unlock(bool w, struct btree *b)
 }
 
 void bch_btree_node_read_done(struct btree *);
+void __bch_btree_node_write(struct btree *, struct closure *);
 void bch_btree_node_write(struct btree *, struct closure *);
 
 void bch_btree_set_root(struct btree *);