configfs: config item dependancies.

Sometimes other drivers depend on particular configfs items.  For
example, ocfs2 mounts depend on a heartbeat region item.  If that
region item is removed with rmdir(2), the ocfs2 mount must BUG or go
readonly.  Not happy.

This provides two additional API calls: configfs_depend_item() and
configfs_undepend_item().  A client driver can call
configfs_depend_item() on an existing item to tell configfs that it is
depended on.  configfs will then return -EBUSY from rmdir(2) for that
item.  When the item is no longer depended on, the client driver calls
configfs_undepend_item() on it.

These API cannot be called underneath any configfs callbacks, as
they will conflict.  They can block and allocate.  A client driver
probably shouldn't calling them of its own gumption.  Rather it should
be providing an API that external subsystems call.

How does this work?  Imagine the ocfs2 mount process.  When it mounts,
it asks for a heart region item.  This is done via a call into the
heartbeat code.  Inside the heartbeat code, the region item is looked
up.  Here, the heartbeat code calls configfs_depend_item().  If it
succeeds, then heartbeat knows the region is safe to give to ocfs2.
If it fails, it was being torn down anyway, and heartbeat can gracefully
pass up an error.

[ Fixed some bad whitespace in configfs.txt. --Mark ]

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h
index 7b48c034b312..3b0185fdf9a4 100644
--- a/fs/configfs/configfs_internal.h
+++ b/fs/configfs/configfs_internal.h
@@ -29,10 +29,11 @@
 
 struct configfs_dirent {
         atomic_t                s_count;
+        int                     s_dependent_count;
         struct list_head        s_sibling;
         struct list_head        s_children;
         struct list_head        s_links;
-        void                    * s_element;
+        void                    * s_element;
         int                     s_type;
         umode_t                 s_mode;
         struct dentry           * s_dentry;
@@ -41,8 +42,8 @@ struct configfs_dirent {
 
 #define CONFIGFS_ROOT           0x0001
 #define CONFIGFS_DIR            0x0002
-#define CONFIGFS_ITEM_ATTR      0x0004
-#define CONFIGFS_ITEM_LINK      0x0020
+#define CONFIGFS_ITEM_ATTR      0x0004
+#define CONFIGFS_ITEM_LINK      0x0020
 #define CONFIGFS_USET_DIR       0x0040
 #define CONFIGFS_USET_DEFAULT   0x0080
 #define CONFIGFS_USET_DROPPING  0x0100