[SRU oem-5.6] bcache: fix potential deadlock problem in btree_gc_coalesce

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Wed Sep 16 20:24:57 UTC 2020

From: Zhiqiang Liu <liuzhiqiang26 at huawei.com>

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
	// main coalescing process
	for (i = nodes - 1; i > 0; --i)
		if coalescing process fails:
			// Here, directly goto out_nocoalesce
			 // tag will cause a deadlock
			goto out_nocoalesce;
	// release new_nodes[i]->write_lock
	// coalesing succ, return
	btree_node_free(new_nodes[i])	// free new_nodes[i]
	// obtain new_nodes[i]->write_lock
	// set flag for reuse
	clear_bit(BTREE_NODE_dirty, &ew_nodes[i]->flags);
	// release 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 at huawei.com>
Signed-off-by: Coly Li <colyli at suse.de>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
(cherry picked from commit be23e837333a914df3f24bf0b32e87b0331ab8d1)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
 drivers/md/bcache/btree.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index fa872df4e770..cad8b0b97e33 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1447,7 +1447,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
 			if (__set_blocks(n1, n1->keys + n2->keys,
 					 block_bytes(b->c)) >
-				goto out_nocoalesce;
+				goto out_unlock_nocoalesce;
 			keys = n2->keys;
 			/* Take the key of the node we're getting rid of */
@@ -1476,7 +1476,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
 		if (__bch_keylist_realloc(&keylist,
-			goto out_nocoalesce;
+			goto out_unlock_nocoalesce;
 		bch_btree_node_write(new_nodes[i], &cl);
 		bch_keylist_add(&keylist, &new_nodes[i]->key);
@@ -1522,6 +1522,10 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
 	/* Invalidated our iterator */
 	return -EINTR;
+	for (i = 0; i < nodes; i++)
+		mutex_unlock(&new_nodes[i]->write_lock);

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