[PATCH 2/3] fs: warn when mount reference count becomes negative

[Sascha Hauer <s.hauer@pengutronix.de>]

Warn when the reference count of a mount would become negative.
With this we warn when it happens. Previously, only unmounting
a mountpoint failed with -EBUSY which was misleading because
mnt->ref was only true because it was negative.

Do not actually allow it to become negative anymore.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
---
 fs/fs.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/fs.c b/fs/fs.c
index f7859304a7b5f4158db83298b586c633668301d5..e063dc48d9d53a64932564c4b983d5ca1b026a1a 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -260,6 +260,12 @@ static void mntput(struct vfsmount *mnt)
 	if (!mnt)
 		return;
 
+	if (!mnt->ref) {
+		WARN_ONCE(1, "refcount for mount \"%s\" becomes negative\n",
+			  mnt->mountpoint->name);
+		return;
+	}
+
 	mnt->ref--;
 }
 

-- 
2.47.3