[PATCH v9 9/9] btrfs: Use sb_want_write() to protect sysfs feature change.

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Just like label change, use sb_want_write() to do a correct protection.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
v4:
  Newly introduced.
v5:
  Change to sb_want_write().
v6:
  Move sb_want_write() to the beginning of the function.
v7:
  None
v8:
  Move sb_want_write() after get fs_info.
v9:
  Fix a unpaired sb_drop_write() in error handler.
---
 fs/btrfs/sysfs.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 7e548f7..19876ba 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c
@@ -121,10 +121,14 @@ static ssize_t btrfs_feature_attr_store(struct kobject *kobj,
 	if (!fs_info)
 		return -EPERM;
 
-	ret = kstrtoul(skip_spaces(buf), 0, &val);
+	ret = sb_want_write(fs_info->sb);
 	if (ret)
 		return ret;
 
+	ret = kstrtoul(skip_spaces(buf), 0, &val);
+	if (ret)
+		goto out;
+
 	if (fa->feature_set == FEAT_COMPAT) {
 		set = BTRFS_FEATURE_COMPAT_SAFE_SET;
 		clear = BTRFS_FEATURE_COMPAT_SAFE_CLEAR;
@@ -141,22 +145,25 @@ static ssize_t btrfs_feature_attr_store(struct kobject *kobj,
 	/* Nothing to do */
 	if ((val && (features & fa->feature_bit)) ||
 	    (!val && !(features & fa->feature_bit)))
-		return count;
+		goto out;
 
 	if ((val && !(set & fa->feature_bit)) ||
 	    (!val && !(clear & fa->feature_bit))) {
 		btrfs_info(fs_info,
 			"%sabling feature %s on mounted fs is not supported.",
 			val ? "En" : "Dis", fa->kobj_attr.attr.name);
-		return -EPERM;
+		ret = -EPERM;
+		goto out;
 	}
 
 	btrfs_info(fs_info, "%s %s feature flag",
 		   val ? "Setting" : "Clearing", fa->kobj_attr.attr.name);
 
 	trans = btrfs_start_transaction(fs_info->fs_root, 0);
-	if (IS_ERR(trans))
-		return PTR_ERR(trans);
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		goto out;
+	}
 
 	spin_lock(&fs_info->super_lock);
 	features = get_features(fs_info, fa->feature_set);
@@ -168,10 +175,12 @@ static ssize_t btrfs_feature_attr_store(struct kobject *kobj,
 	spin_unlock(&fs_info->super_lock);
 
 	ret = btrfs_commit_transaction(trans, fs_info->fs_root);
-	if (ret)
-		return ret;
 
-	return count;
+out:
+	sb_drop_write(fs_info->sb);
+	if (!ret)
+		return count;
+	return ret;
 }
 
 static umode_t btrfs_feature_visible(struct kobject *kobj,
-- 
2.3.0