15 files changed, 126 insertions, 50 deletions

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 5f0c52a07386..cbf98b989b11 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -20,6 +20,7 @@ prototypes:
 	void (*d_iput)(struct dentry *, struct inode *);
 	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 	struct vfsmount *(*d_automount)(struct path *path);
+	int (*d_manage)(struct dentry *, bool);
 
 locking rules:
 		rename_lock	->d_lock	may block	rcu-walk
@@ -31,6 +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
 
 --------------------------- inode_operations --------------------------- 
 prototypes:
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 726a4f6fa3c9..4682586b147a 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -865,6 +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);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -938,12 +939,30 @@ struct dentry_operations {
 	target and the parent VFS mount record to provide inheritable mount
 	parameters.  NULL should be returned if someone else managed to make
 	the automount first.  If the automount failed, then an error code
-	should be returned.
+	should be returned.  If -EISDIR is returned, then the directory will
+	be treated as an ordinary directory and returned to pathwalk to
+	continue walking.
 
 	This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
 	dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
 	inode being added.
 
+  d_manage: called to allow the filesystem to manage the transition from a
+	dentry (optional).  This allows autofs, for example, to hold up clients
+	waiting to explore behind a 'mountpoint' whilst letting the daemon go
+	past and construct the subtree there.  0 should be returned to let the
+	calling process continue.  -EISDIR can be returned to tell pathwalk to
+	use this directory as an ordinary directory and to ignore anything
+	mounted on it and not to check the automount flag.  Any other error
+	code will abort pathwalk completely.
+
+	If the 'mounting_here' parameter is true, then namespace_sem is being
+	held by the caller and the function should not initiate any mounts or
+	unmounts that it will then wait for.
+
+	This function is only used if DCACHE_MANAGE_TRANSIT is set on the
+	dentry being transited from.
+
 Example :
 
 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
diff --git a/drivers/staging/autofs/dirhash.c b/drivers/staging/autofs/dirhash.c
index d3f42c8325f7..a08bd7355035 100644
--- a/drivers/staging/autofs/dirhash.c
+++ b/drivers/staging/autofs/dirhash.c
@@ -88,14 +88,13 @@ struct autofs_dir_ent *autofs_expire(struct super_block *sb,
 		}
 		path.mnt = mnt;
 		path_get(&path);
-		if (!follow_down(&path)) {
+		if (!follow_down_one(&path)) {
 			path_put(&path);
 			DPRINTK(("autofs: not expirable\
 			(not a mounted directory): %s\n", ent->name));
 			continue;
 		}
-		while (d_mountpoint(path.dentry) && follow_down(&path))
-			;
+		follow_down(&path, false);  // TODO: need to check error
 		umount_ok = may_umount(path.mnt);
 		path_put(&path);
 
diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index e83c0336e7b5..f3e891d57a2c 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -273,10 +273,7 @@ static void *afs_mntpt_follow_link(struct dentry *dentry, struct nameidata *nd)
 		break;
 	case -EBUSY:
 		/* someone else made a mount here whilst we were busy */
-		while (d_mountpoint(nd->path.dentry) &&
-		       follow_down(&nd->path))
-			;
-		err = 0;
+		err = follow_down(&nd->path, false);
 	default:
 		mntput(newmnt);
 		break;
diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index 0fffe1c24cec..eb67953452bb 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -229,19 +229,6 @@ int autofs4_wait(struct autofs_sb_info *,struct dentry *, enum autofs_notify);
 int autofs4_wait_release(struct autofs_sb_info *,autofs_wqt_t,int);
 void autofs4_catatonic_mode(struct autofs_sb_info *);
 
-static inline int autofs4_follow_mount(struct path *path)
-{
-	int res = 0;
-
-	while (d_mountpoint(path->dentry)) {
-		int followed = follow_down(path);
-		if (!followed)
-			break;
-		res = 1;
-	}
-	return res;
-}
-
 static inline u32 autofs4_get_dev(struct autofs_sb_info *sbi)
 {
 	return new_encode_dev(sbi->sb->s_dev);
diff --git a/fs/autofs4/dev-ioctl.c b/fs/autofs4/dev-ioctl.c
index eff9a419469a..1442da4860e5 100644
--- a/fs/autofs4/dev-ioctl.c
+++ b/fs/autofs4/dev-ioctl.c
@@ -551,7 +551,7 @@ static int autofs_dev_ioctl_ismountpoint(struct file *fp,
 
 		err = have_submounts(path.dentry);
 
-		if (follow_down(&path))
+		if (follow_down_one(&path))
 			magic = path.mnt->mnt_sb->s_magic;
 	}
 
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index cc1d01365905..6a930b90d389 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -56,7 +56,7 @@ static int autofs4_mount_busy(struct vfsmount *mnt, struct dentry *dentry)
 
 	path_get(&path);
 
-	if (!follow_down(&path))
+	if (!follow_down_one(&path))
 		goto done;
 
 	if (is_autofs4_dentry(path.dentry)) {
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 651e4ef563b1..20225636a4e9 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -234,7 +234,7 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
 		nd->flags);
 	/*
 	 * For an expire of a covered direct or offset mount we need
-	 * to break out of follow_down() at the autofs mount trigger
+	 * to break out of follow_down_one() at the autofs mount trigger
 	 * (d_mounted--), so we can see the expiring flag, and manage
 	 * the blocking and following here until the expire is completed.
 	 */
@@ -243,7 +243,7 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
 		if (ino->flags & AUTOFS_INF_EXPIRING) {
 			spin_unlock(&sbi->fs_lock);
 			/* Follow down to our covering mount. */
-			if (!follow_down(&nd->path))
+			if (!follow_down_one(&nd->path))
 				goto done;
 			goto follow;
 		}
@@ -292,11 +292,10 @@ follow:
 	 * multi-mount with no root offset so we don't need
 	 * to follow it.
 	 */
-	if (d_mountpoint(dentry)) {
-		if (!autofs4_follow_mount(&nd->path)) {
-			status = -ENOENT;
+	if (d_managed(dentry)) {
+		status = follow_down(&nd->path, false);
+		if (status < 0)
 			goto out_error;
-		}
 	}
 
 done:
diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
index c68a056f27fd..83479cf63f96 100644
--- a/fs/cifs/cifs_dfs_ref.c
+++ b/fs/cifs/cifs_dfs_ref.c
@@ -273,10 +273,7 @@ static int add_mount_helper(struct vfsmount *newmnt, struct nameidata *nd,
 		break;
 	case -EBUSY:
 		/* someone else made a mount here whilst we were busy */
-		while (d_mountpoint(nd->path.dentry) &&
-		       follow_down(&nd->path))
-			;
-		err = 0;
+		err = follow_down(&nd->path, false);
 	default:
 		mntput(newmnt);
 		break;
diff --git a/fs/namei.c b/fs/namei.c
index 16109da68bbf..9d3033dc22e9 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -960,6 +960,7 @@ static int follow_automount(struct path *path, unsigned flags,
 
 /*
  * Handle a dentry that is managed in some way.
+ * - Flagged for transit management (autofs)
  * - Flagged as mountpoint
  * - Flagged as automount point
  *
@@ -979,6 +980,16 @@ static int follow_managed(struct path *path, unsigned flags)
 	while (managed = ACCESS_ONCE(path->dentry->d_flags),
 	       managed &= DCACHE_MANAGED_DENTRY,
 	       unlikely(managed != 0)) {
+		/* Allow the filesystem to manage the transit without i_mutex
+		 * being held. */
+		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);
+			if (ret < 0)
+				return ret == -EISDIR ? 0 : ret;
+		}
+
 		/* Transit to a mounted filesystem. */
 		if (managed & DCACHE_MOUNTED) {
 			struct vfsmount *mounted = lookup_mnt(path);
@@ -1012,7 +1023,7 @@ static int follow_managed(struct path *path, unsigned flags)
 	return 0;
 }
 
-int follow_down(struct path *path)
+int follow_down_one(struct path *path)
 {
 	struct vfsmount *mounted;
 
@@ -1029,14 +1040,19 @@ int follow_down(struct path *path)
 
 /*
  * Skip to top of mountpoint pile in rcuwalk mode.  We abort the rcu-walk if we
- * meet an automount point and we're not walking to "..".  True is returned to
+ * meet a managed dentry and we're not walking to "..".  True is returned to
  * continue, false to abort.
  */
 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)
+			return true;
 		mounted = __lookup_mnt(path->mnt, path->dentry, 1);
 		if (!mounted)
 			break;
@@ -1087,6 +1103,57 @@ static int follow_dotdot_rcu(struct nameidata *nd)
 }
 
 /*
+ * Follow down to the covering mount currently visible to userspace.  At each
+ * point, the filesystem owning that dentry may be queried as to whether the
+ * caller is permitted to proceed or not.
+ *
+ * Care must be taken as namespace_sem may be held (indicated by mounting_here
+ * being true).
+ */
+int follow_down(struct path *path, bool mounting_here)
+{
+	unsigned managed;
+	int ret;
+
+	while (managed = ACCESS_ONCE(path->dentry->d_flags),
+	       unlikely(managed & DCACHE_MANAGED_DENTRY)) {
+		/* Allow the filesystem to manage the transit without i_mutex
+		 * being held.
+		 *
+		 * We indicate to the filesystem if someone is trying to mount
+		 * something here.  This gives autofs the chance to deny anyone
+		 * other than its daemon the right to mount on its
+		 * superstructure.
+		 *
+		 * The filesystem may sleep at this point.
+		 */
+		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);
+			if (ret < 0)
+				return ret == -EISDIR ? 0 : ret;
+		}
+
+		/* Transit to a mounted filesystem. */
+		if (managed & DCACHE_MOUNTED) {
+			struct vfsmount *mounted = lookup_mnt(path);
+			if (!mounted)
+				break;
+			dput(path->dentry);
+			mntput(path->mnt);
+			path->mnt = mounted;
+			path->dentry = dget(mounted->mnt_root);
+			continue;
+		}
+
+		/* Don't handle automount points here */
+		break;
+	}
+	return 0;
+}
+
+/*
  * Skip to top of mountpoint pile in refwalk mode for follow_dotdot()
  */
 static void follow_mount(struct path *path)
@@ -3530,6 +3597,7 @@ const struct inode_operations page_symlink_inode_operations = {
 };
 
 EXPORT_SYMBOL(user_path_at);
+EXPORT_SYMBOL(follow_down_one);
 EXPORT_SYMBOL(follow_down);
 EXPORT_SYMBOL(follow_up);
 EXPORT_SYMBOL(get_write_access); /* binfmt_aout */
diff --git a/fs/namespace.c b/fs/namespace.c
index 3ddfd9046c44..d94ccd6ddafd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1844,9 +1844,10 @@ static int do_move_mount(struct path *path, char *old_name)
 		return err;
 
 	down_write(&namespace_sem);
-	while (d_mountpoint(path->dentry) &&
-	       follow_down(path))
-		;
+	err = follow_down(path, true);
+	if (err < 0)
+		goto out;
+
 	err = -EINVAL;
 	if (!check_mnt(path->mnt) || !check_mnt(old_path.mnt))
 		goto out;
@@ -1940,9 +1941,10 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
 
 	down_write(&namespace_sem);
 	/* Something was mounted here while we slept */
-	while (d_mountpoint(path->dentry) &&
-	       follow_down(path))
-		;
+	err = follow_down(path, true);
+	if (err < 0)
+		goto unlock;
+
 	err = -EINVAL;
 	if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))
 		goto unlock;
diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index 74aaf3963c10..bfcb933e5755 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -176,10 +176,7 @@ out_err:
 	path_put(&nd->path);
 	goto out;
 out_follow:
-	while (d_mountpoint(nd->path.dentry) &&
-	       follow_down(&nd->path))
-		;
-	err = 0;
+	err = follow_down(&nd->path, false);
 	goto out;
 }
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 230b79fbf005..0f79e33a65d7 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -88,8 +88,9 @@ nfsd_cross_mnt(struct svc_rqst *rqstp, struct dentry **dpp,
 			    .dentry = dget(dentry)};
 	int err = 0;
 
-	while (d_mountpoint(path.dentry) && follow_down(&path))
-		;
+	err = follow_down(&path, false);
+	if (err < 0)
+		goto out;
 
 	exp2 = rqst_exp_get_by_name(rqstp, &path);
 	if (IS_ERR(exp2)) {
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index ee6c26d142c3..1a87760d6532 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -168,6 +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);
 } ____cacheline_aligned;
 
 /*
@@ -214,8 +215,9 @@ struct dentry_operations {
 
 #define DCACHE_MOUNTED		0x10000	/* is a mountpoint */
 #define DCACHE_NEED_AUTOMOUNT	0x20000	/* handle automount on this dir */
+#define DCACHE_MANAGE_TRANSIT	0x40000	/* manage transit from this dirent */
 #define DCACHE_MANAGED_DENTRY \
-	(DCACHE_MOUNTED|DCACHE_NEED_AUTOMOUNT)
+	(DCACHE_MOUNTED|DCACHE_NEED_AUTOMOUNT|DCACHE_MANAGE_TRANSIT)
 
 extern seqlock_t rename_lock;
 
@@ -404,7 +406,12 @@ static inline void dont_mount(struct dentry *dentry)
 
 extern void dput(struct dentry *);
 
-static inline int d_mountpoint(struct dentry *dentry)
+static inline bool d_managed(struct dentry *dentry)
+{
+	return dentry->d_flags & DCACHE_MANAGED_DENTRY;
+}
+
+static inline bool d_mountpoint(struct dentry *dentry)
 {
 	return dentry->d_flags & DCACHE_MOUNTED;
 }
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 18d06add0a40..8ef2c789c2a8 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -79,7 +79,8 @@ extern struct file *lookup_instantiate_filp(struct nameidata *nd, struct dentry
 
 extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
 
-extern int follow_down(struct path *);
+extern int follow_down_one(struct path *);
+extern int follow_down(struct path *, bool);
 extern int follow_up(struct path *);
 
 extern struct dentry *lock_rename(struct dentry *, struct dentry *);