FAILED: patch "[PATCH] btrfs: avoid double put of block group when emptying cluster" failed to apply to 4.19-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 95c85fba1f64c3249c67f0078a29f8a125078189 Mon Sep 17 00:00:00 2001
From: Josef Bacik <josef@toxicpanda.com>
Date: Mon, 25 Jan 2021 16:42:35 -0500
Subject: [PATCH] btrfs: avoid double put of block group when emptying cluster

It's wrong calling btrfs_put_block_group in
__btrfs_return_cluster_to_free_space if the block group passed is
different than the block group the cluster represents. As this means the
cluster doesn't have a reference to the passed block group. This results
in double put and a use-after-free bug.

Fix this by simply bailing if the block group we passed in does not
match the block group on the cluster.

Fixes: fa9c0d795f7b ("Btrfs: rework allocation clustering")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index abcf951e6b44..711a6a751ae9 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -2801,8 +2801,10 @@ static void __btrfs_return_cluster_to_free_space(
 	struct rb_node *node;
 
 	spin_lock(&cluster->lock);
-	if (cluster->block_group != block_group)
-		goto out;
+	if (cluster->block_group != block_group) {
+		spin_unlock(&cluster->lock);
+		return;
+	}
 
 	cluster->block_group = NULL;
 	cluster->window_start = 0;
@@ -2840,8 +2842,6 @@ static void __btrfs_return_cluster_to_free_space(
 				   entry->offset, &entry->offset_index, bitmap);
 	}
 	cluster->root = RB_ROOT;
-
-out:
 	spin_unlock(&cluster->lock);
 	btrfs_put_block_group(block_group);
 }