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, 7 insertions, 2 deletions

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index ddfde380b49f..9ded06434e11 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1398,9 +1398,12 @@ static void cache_set_flush(struct closure *cl)
 		list_add(&c->root->list, &c->btree_cache);
 
 	/* Should skip this if we're unregistering because of an error */
-	list_for_each_entry(b, &c->btree_cache, list)
+	list_for_each_entry(b, &c->btree_cache, list) {
+		mutex_lock(&b->write_lock);
 		if (btree_node_dirty(b))
-			bch_btree_node_write(b, NULL);
+			__bch_btree_node_write(b, NULL);
+		mutex_unlock(&b->write_lock);
+	}
 
 	for_each_cache(ca, c, i)
 		if (ca->alloc_thread)
@@ -1667,8 +1670,10 @@ static void run_cache_set(struct cache_set *c)
 		if (IS_ERR_OR_NULL(c->root))
 			goto err;
 
+		mutex_lock(&c->root->write_lock);
 		bkey_copy_key(&c->root->key, &MAX_KEY);
 		bch_btree_node_write(c->root, &cl);
+		mutex_unlock(&c->root->write_lock);
 
 		bch_btree_set_root(c->root);
 		rw_unlock(true, c->root);