fs: add mnt_allow_writers() and simplify mount_setattr_prepare()

Add a tiny helper that lets us simplify the control-flow and can be used
in the next patch to avoid adding another condition open-coded into
mount_setattr_prepare(). Instead we can add it into the new helper.

Link: https://lore.kernel.org/r/20220203131411.3093040-5-brauner@kernel.org
Cc: Seth Forshee <seth.forshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index de6fae84f1a1ab2d583d752c9d28bfb7e49eefcf..7e5535ed155d9868c489bfe618a129b56ad0262c 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3998,6 +3998,22 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
        return 0;
 }
 
+/**
+ * mnt_allow_writers() - check whether the attribute change allows writers
+ * @kattr: the new mount attributes
+ * @mnt: the mount to which @kattr will be applied
+ *
+ * Check whether thew new mount attributes in @kattr allow concurrent writers.
+ *
+ * Return: true if writers need to be held, false if not
+ */
+static inline bool mnt_allow_writers(const struct mount_kattr *kattr,
+                                    const struct mount *mnt)
+{
+       return !(kattr->attr_set & MNT_READONLY) ||
+              (mnt->mnt.mnt_flags & MNT_READONLY);
+}
+
 static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
                                           struct mount *mnt, int *err)
 {
@@ -4028,12 +4044,12 @@ static struct mount *mount_setattr_prepare(struct mount_kattr *kattr,
 
                last = m;
 
-               if ((kattr->attr_set & MNT_READONLY) &&
-                   !(m->mnt.mnt_flags & MNT_READONLY)) {
-                       *err = mnt_hold_writers(m);
-                       if (*err)
-                               goto out;
-               }
+               if (mnt_allow_writers(kattr, m))
+                       continue;
+
+               *err = mnt_hold_writers(m);
+               if (*err)
+                       goto out;
        } while (kattr->recurse && (m = next_mnt(m, mnt)));
 
 out: