add several pieces to shared subtree documentation

* document locking
* add the missing part of data structure invariants (relationship
between mnt_share and mnt_slave lists in case of a peer group
among slaves).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/Documentation/filesystems/sharedsubtree.txt b/Documentation/filesystems/sharedsubtree.txt
index 23a181074f94b5f65ac25c603d65fc749fa5ed78..fc0e39af43c32a42343f97daed5a5d285f844863 100644 (file)
--- a/Documentation/filesystems/sharedsubtree.txt
+++ b/Documentation/filesystems/sharedsubtree.txt
@@ -837,6 +837,9 @@ replicas continue to be exactly same.
         individual lists does not affect propagation or the way propagation
         tree is modified by operations.
 
         individual lists does not affect propagation or the way propagation
         tree is modified by operations.
 

+       All vfsmounts in a peer group have the same ->mnt_master.  If it is
+       non-NULL, they form a contiguous (ordered) segment of slave list.
+

        A example propagation tree looks as shown in the figure below.
        [ NOTE: Though it looks like a forest, if we consider all the shared
        mounts as a conceptual entity called 'pnode', it becomes a tree]
        A example propagation tree looks as shown in the figure below.
        [ NOTE: Though it looks like a forest, if we consider all the shared
        mounts as a conceptual entity called 'pnode', it becomes a tree]


@@ -874,8 +877,19 @@ replicas continue to be exactly same.
 
        NOTE: The propagation tree is orthogonal to the mount tree.
 
 
        NOTE: The propagation tree is orthogonal to the mount tree.
 

+8B Locking:
+
+       ->mnt_share, ->mnt_slave, ->mnt_slave_list, ->mnt_master are protected
+       by namespace_sem (exclusive for modifications, shared for reading).
+
+       Normally we have ->mnt_flags modifications serialized by vfsmount_lock.
+       There are two exceptions: do_add_mount() and clone_mnt().
+       The former modifies a vfsmount that has not been visible in any shared
+       data structures yet.
+       The latter holds namespace_sem and the only references to vfsmount
+       are in lists that can't be traversed without namespace_sem.

 
 

-8B Algorithm:
+8C Algorithm:

 
        The crux of the implementation resides in rbind/move operation.
 
 
        The crux of the implementation resides in rbind/move operation.