[PATCH v2] overlayfs.rst: clarify the scope of file locks taken through the overlay

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

File locks and leases taken on files through the overlay mount are
only meaningful between tasks that access the files through the same
overlay mount; they do not conflict with anything that changes the
underlying filesystems directly.  Add a short note to "Changes to
underlying filesystems" stating this scope.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
Changes since v1:
- minimal note within "Changes to underlying filesystems" instead of a
  separate section, per review (Amir Goldstein)
---
 Documentation/filesystems/overlayfs.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 16c35b491dad..11af5bc73e32 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -671,6 +671,15 @@ filesystem are not allowed.  If the underlying filesystem is changed,
 the behavior of the overlay is undefined, though it will not result in
 a crash or deadlock.
 
+Note that file locks and leases taken on files through the overlay
+mount only provide mutual exclusion between tasks that access the
+files through the same overlay mount, and a lease taken through the
+overlay is broken by opens through that overlay.  They do not
+conflict with locks taken on the same files by anything that changes
+the underlying filesystems directly, so file locking cannot be relied
+on for mutual exclusion between overlay users and such direct
+changes.
+
 Offline changes, when the overlay is not mounted, are allowed to the
 upper tree.  Offline changes to the lower tree are only allowed if the
 "metacopy", "index", "xino" and "redirect_dir" features
-- 
2.43.0