bcache: fix potential deadlock problem in btree_gc_coalesce

coccicheck reports:
  drivers/md//bcache/btree.c:1538:1-7: preceding lock on line 1417

In btree_gc_coalesce func, if the coalescing process fails, we will goto
to out_nocoalesce tag directly without releasing new_nodes[i]->write_lock.
Then, it will cause a deadlock when trying to acquire new_nodes[i]->
write_lock for freeing new_nodes[i] before return.

btree_gc_coalesce func details as follows:
	if alloc new_nodes[i] fails:
		goto out_nocoalesce;
	// obtain new_nodes[i]->write_lock
	mutex_lock(&new_nodes[i]->write_lock)
	// main coalescing process
	for (i = nodes - 1; i > 0; --i)
		[snipped]
		if coalescing process fails:
			// Here, directly goto out_nocoalesce
			 // tag will cause a deadlock
			goto out_nocoalesce;
		[snipped]
	// release new_nodes[i]->write_lock
	mutex_unlock(&new_nodes[i]->write_lock)
	// coalesing succ, return
	return;
out_nocoalesce:
	btree_node_free(new_nodes[i])	// free new_nodes[i]
	// obtain new_nodes[i]->write_lock
	mutex_lock(&new_nodes[i]->write_lock);
	// set flag for reuse
	clear_bit(BTREE_NODE_dirty, &ew_nodes[i]->flags);
	// release new_nodes[i]->write_lock
	mutex_unlock(&new_nodes[i]->write_lock);

To fix the problem, we add a new tag 'out_unlock_nocoalesce' for
releasing new_nodes[i]->write_lock before out_nocoalesce tag. If
coalescing process fails, we will go to out_unlock_nocoalesce tag
for releasing new_nodes[i]->write_lock before free new_nodes[i] in
out_nocoalesce tag.

(Coly Li helps to clean up commit log format.)

Fixes: 2a285686c109816 ("bcache: btree locking rework")
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 39de94e..6548a60 100644 (file)
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1389,7 +1389,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
                        if (__set_blocks(n1, n1->keys + n2->keys,
                                         block_bytes(b->c)) >
                            btree_blocks(new_nodes[i]))
-                               goto out_nocoalesce;
+                               goto out_unlock_nocoalesce;
 
                        keys = n2->keys;
                        /* Take the key of the node we're getting rid of */
@@ -1418,7 +1418,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
 
                if (__bch_keylist_realloc(&keylist,
                                          bkey_u64s(&new_nodes[i]->key)))
-                       goto out_nocoalesce;
+                       goto out_unlock_nocoalesce;
 
                bch_btree_node_write(new_nodes[i], &cl);
                bch_keylist_add(&keylist, &new_nodes[i]->key);
@@ -1464,6 +1464,10 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
        /* Invalidated our iterator */
        return -EINTR;
 
+out_unlock_nocoalesce:
+       for (i = 0; i < nodes; i++)
+               mutex_unlock(&new_nodes[i]->write_lock);
+
 out_nocoalesce:
        closure_sync(&cl);