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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>
This commit is contained in:
Al Viro 2010-01-16 14:10:21 -05:00
parent 796a6b521d
commit 2f99cc6e46
1 changed files with 15 additions and 1 deletions
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16
Documentation/filesystems/sharedsubtree.txt
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@ -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.
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]
@ -874,8 +877,19 @@ replicas continue to be exactly same.
NOTE: The propagation tree is orthogonal to the mount tree.
8B Locking:
8B Algorithm:
->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.
8C Algorithm:
The crux of the implementation resides in rbind/move operation.