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>

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:

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.
 

fs/autofs4/root.c

@@ -36,7 +36,7 @@ static long autofs4_root_compat_ioctl(struct file *,unsigned int,unsigned long);
 static int autofs4_dir_open(struct inode *inode, struct file *file);
 static struct dentry *autofs4_lookup(struct inode *,struct dentry *, struct nameidata *);
 static struct vfsmount *autofs4_d_automount(struct path *);
-static int autofs4_d_manage(struct dentry *, bool);
+static int autofs4_d_manage(struct dentry *, bool, bool);
 
 const struct file_operations autofs4_root_operations = {
 	.open		= dcache_dir_open,
@@ -450,7 +450,7 @@ static struct vfsmount *autofs4_d_automount(struct path *path)
 	return NULL;
 }
 
-int autofs4_d_manage(struct dentry *dentry, bool mounting_here)
+int autofs4_d_manage(struct dentry *dentry, bool mounting_here, bool rcu_walk)
 {
 	struct autofs_sb_info *sbi = autofs4_sbi(dentry->d_sb);
 
@@ -464,6 +464,10 @@ int autofs4_d_manage(struct dentry *dentry, bool mounting_here)
 		return 0;
 	}
 
+	/* We need to sleep, so we need pathwalk to be in ref-mode */
+	if (rcu_walk)
+		return -ECHILD;
+
 	/* Wait for pending expires */
 	do_expire_wait(dentry);
 

fs/namei.c

@@ -987,7 +987,8 @@ static int follow_managed(struct path *path, unsigned flags)
 		if (managed & DCACHE_MANAGE_TRANSIT) {
 			BUG_ON(!path->dentry->d_op);
 			BUG_ON(!path->dentry->d_op->d_manage);
-			ret = path->dentry->d_op->d_manage(path->dentry, false);
+			ret = path->dentry->d_op->d_manage(path->dentry,
+							   false, false);
 			if (ret < 0)
 				return ret == -EISDIR ? 0 : ret;
 		}
@@ -1048,13 +1049,12 @@ int follow_down_one(struct path *path)
 static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 			       struct inode **inode, bool reverse_transit)
 {
-	unsigned abort_mask =
-		reverse_transit ? 0 : DCACHE_MANAGE_TRANSIT;
-
 	while (d_mountpoint(path->dentry)) {
 		struct vfsmount *mounted;
-		if (path->dentry->d_flags & abort_mask)
+		if (unlikely(path->dentry->d_flags & DCACHE_MANAGE_TRANSIT) &&
-			return true;
+		    !reverse_transit &&
+		    path->dentry->d_op->d_manage(path->dentry, false, true) < 0)
+			return false;
 		mounted = __lookup_mnt(path->mnt, path->dentry, 1);
 		if (!mounted)
 			break;
@@ -1132,7 +1132,8 @@ int follow_down(struct path *path, bool mounting_here)
 		if (managed & DCACHE_MANAGE_TRANSIT) {
 			BUG_ON(!path->dentry->d_op);
 			BUG_ON(!path->dentry->d_op->d_manage);
-			ret = path->dentry->d_op->d_manage(path->dentry, mounting_here);
+			ret = path->dentry->d_op->d_manage(
+				path->dentry, mounting_here, false);
 			if (ret < 0)
 				return ret == -EISDIR ? 0 : ret;
 		}

include/linux/dcache.h

@@ -168,7 +168,7 @@ 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);
 } ____cacheline_aligned;
 
 /*