linux-kernel - [PATCH 5.4 266/314] bcache: fix potential deadlock problem in btree_gc

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Message-Id: <20200623195351.663361083@linuxfoundation.org>
Date:   Tue, 23 Jun 2020 21:57:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Zhiqiang Liu <liuzhiqiang26@...wei.com>,
        Coly Li <colyli@...e.de>, Jens Axboe <axboe@...nel.dk>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.4 266/314] bcache: fix potential deadlock problem in btree_gc_coalesce

From: Zhiqiang Liu <liuzhiqiang26@...wei.com>

[ Upstream commit be23e837333a914df3f24bf0b32e87b0331ab8d1 ]

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@...wei.com>
Signed-off-by: Coly Li <colyli@...e.de>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 46a8b5a91c386..3c1109fceb2fb 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -1442,7 +1442,7 @@ 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 */
@@ -1471,7 +1471,7 @@ 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);
@@ -1517,6 +1517,10 @@ 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);
 
-- 
2.25.1