Allow d_manage() to be used in RCU-walk mode

Allow d_manage() to be called from pathwalk when it is in RCU-walk mode as well
as when it is in Ref-walk mode.  This permits __follow_mount_rcu() to call
d_manage() directly.  d_manage() needs a parameter to indicate that it is in
RCU-walk mode as it isn't allowed to sleep if in that mode (but should return
-ECHILD instead).

autofs4_d_manage() can then be set to retain RCU-walk mode if the daemon
accesses it and otherwise request dropping back to ref-walk mode.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 7 insertions, 2 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index cbf98b989b1..39707748ed2 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -32,7 +32,7 @@ d_release:	no		no		yes		no
 d_iput:         no              no              yes             no
 d_dname:        no              no              no              no
 d_automount:    no              no              yes             no
-d_manage:       no              no              yes             no
+d_manage:       no              no              yes (ref-walk)  maybe
 
 --------------------------- inode_operations --------------------------- 
 prototypes:
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 4682586b147..3c4b2f1b64d 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -865,7 +865,7 @@ struct dentry_operations {
         void (*d_iput)(struct dentry *, struct inode *);
         char *(*d_dname)(struct dentry *, char *, int);
         struct vfsmount *(*d_automount)(struct path *);
-        int (*d_manage)(struct dentry *, bool);
+        int (*d_manage)(struct dentry *, bool, bool);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -960,6 +960,11 @@ struct dentry_operations {
         held by the caller and the function should not initiate any mounts or
         unmounts that it will then wait for.
 
+        If the 'rcu_walk' parameter is true, then the caller is doing a
+        pathwalk in RCU-walk mode.  Sleeping is not permitted in this mode,
+        and the caller can be asked to leave it and call again by returing
+        -ECHILD.
+
         This function is only used if DCACHE_MANAGE_TRANSIT is set on the
         dentry being transited from.