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, 5 insertions, 0 deletions

diff --git a/include/linux/configfs.h b/include/linux/configfs.h
index 8227e730dac7..8c6967f3fb11 100644
--- a/include/linux/configfs.h
+++ b/include/linux/configfs.h
@@ -188,6 +188,11 @@ static inline struct configfs_subsystem *to_configfs_subsystem(struct config_gro
 int configfs_register_subsystem(struct configfs_subsystem *subsys);
 void configfs_unregister_subsystem(struct configfs_subsystem *subsys);
 
+/* These functions can sleep and can alloc with GFP_KERNEL */
+/* WARNING: These cannot be called underneath configfs callbacks!! */
+int configfs_depend_item(struct configfs_subsystem *subsys, struct config_item *target);
+void configfs_undepend_item(struct configfs_subsystem *subsys, struct config_item *target);
+
 #endif  /* __KERNEL__ */
 
 #endif /* _CONFIGFS_H_ */