replace ->follow_link() with new method that could stay in RCU mode

new method: ->get_link(); replacement of ->follow_link().  The differences
are:
	* inode and dentry are passed separately
	* might be called both in RCU and non-RCU mode;
the former is indicated by passing it a NULL dentry.
	* when called that way it isn't allowed to block
and should return ERR_PTR(-ECHILD) if it needs to be called
in non-RCU mode.

It's a flagday change - the old method is gone, all in-tree instances
converted.  Conversion isn't hard; said that, so far very few instances
do not immediately bail out when called in RCU mode.  That'll change
in the next commits.

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

2 files changed, 8 insertions, 2 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 06d443450f21..4fba54b9fcec 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -50,7 +50,7 @@ prototypes:
         int (*rename2) (struct inode *, struct dentry *,
                         struct inode *, struct dentry *, unsigned int);
         int (*readlink) (struct dentry *, char __user *,int);
-        const char *(*follow_link) (struct dentry *, void **);
+        const char *(*get_link) (struct dentry *, struct inode *, void **);
         void (*put_link) (struct inode *, void *);
         void (*truncate) (struct inode *);
         int (*permission) (struct inode *, int, unsigned int);
@@ -83,7 +83,7 @@ rmdir:		yes (both)	(see below)
 rename:         yes (all)       (see below)
 rename2:        yes (all)       (see below)
 readlink:       no
-follow_link:    no
+get_link:       no
 put_link:       no
 setattr:        yes
 permission:     no (may not block if called in rcu-walk mode)
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 3eb7c35c9698..cf92a8c55594 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -509,3 +509,9 @@ in your dentry operations instead.
         any symlink that might use page_follow_link_light/page_put_link() must
         have inode_nohighmem(inode) called before anything might start playing with
         its pagecache.
+--
+[mandatory]
+        ->follow_link() is replaced with ->get_link(); same API, except that
+                * ->get_link() gets inode as a separate argument
+                * ->get_link() may be called in RCU mode - in that case NULL
+                  dentry is passed