ovl: narrow locking in ovl_whiteout()

ovl_whiteout() relies on the workdir i_rwsem to provide exclusive access
to ofs->whiteout which it manipulates.  Rather than depending on this,
add a new mutex, "whiteout_lock" to explicitly provide the required
locking.  Use guard(mutex) for this so that we can return without
needing to explicitly unlock.

Then take the lock on workdir only when needed - to lookup the temp name
and to do the whiteout or link.

Signed-off-by: NeilBrown <neil@brown.name>
Link: https://lore.kernel.org/20250716004725.1206467-19-neil@brown.name
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/overlayfs/dir.c

@@ -84,41 +84,45 @@ static struct dentry *ovl_whiteout(struct ovl_fs *ofs)
 	struct dentry *workdir = ofs->workdir;
 	struct inode *wdir = workdir->d_inode;
 
-	inode_lock_nested(wdir, I_MUTEX_PARENT);
+	guard(mutex)(&ofs->whiteout_lock);
-	if (!ofs->whiteout) {
-		whiteout = ovl_lookup_temp(ofs, workdir);
-		if (IS_ERR(whiteout))
-			goto out;
 
+	if (!ofs->whiteout) {
+		inode_lock_nested(wdir, I_MUTEX_PARENT);
+		whiteout = ovl_lookup_temp(ofs, workdir);
+		if (!IS_ERR(whiteout)) {
 			err = ovl_do_whiteout(ofs, wdir, whiteout);
 			if (err) {
 				dput(whiteout);
 				whiteout = ERR_PTR(err);
-			goto out;
 			}
+		}
+		inode_unlock(wdir);
+		if (IS_ERR(whiteout))
+			return whiteout;
 		ofs->whiteout = whiteout;
 	}
 
 	if (!ofs->no_shared_whiteout) {
+		inode_lock_nested(wdir, I_MUTEX_PARENT);
 		whiteout = ovl_lookup_temp(ofs, workdir);
-		if (IS_ERR(whiteout))
+		if (!IS_ERR(whiteout)) {
-			goto out;
-
 			err = ovl_do_link(ofs, ofs->whiteout, wdir, whiteout);
-		if (!err)
+			if (err) {
-			goto out;
+				dput(whiteout);
-
+				whiteout = ERR_PTR(err);
-		if (err != -EMLINK) {
+			}
+		}
+		inode_unlock(wdir);
+		if (!IS_ERR(whiteout))
+			return whiteout;
+		if (PTR_ERR(whiteout) != -EMLINK) {
 			pr_warn("Failed to link whiteout - disabling whiteout inode sharing(nlink=%u, err=%i)\n",
 				ofs->whiteout->d_inode->i_nlink, err);
 			ofs->no_shared_whiteout = true;
 		}
-		dput(whiteout);
 	}
 	whiteout = ofs->whiteout;
 	ofs->whiteout = NULL;
-out:
-	inode_unlock(wdir);
 	return whiteout;
 }
 

fs/overlayfs/ovl_entry.h

@@ -88,6 +88,7 @@ struct ovl_fs {
 	/* Shared whiteout cache */
 	struct dentry *whiteout;
 	bool no_shared_whiteout;
+	struct mutex whiteout_lock;
 	/* r/o snapshot of upperdir sb's only taken on volatile mounts */
 	errseq_t errseq;
 };

fs/overlayfs/params.c

@@ -795,6 +795,8 @@ int ovl_init_fs_context(struct fs_context *fc)
 	fc->s_fs_info		= ofs;
 	fc->fs_private		= ctx;
 	fc->ops			= &ovl_context_ops;
+
+	mutex_init(&ofs->whiteout_lock);
 	return 0;
 
 out_err: