Fix the regression created by "set S_DEAD on unlink()..." commit

1) i_flags simply doesn't work for mount/unlink race prevention;
we may have many links to file and rm on one of those obviously
shouldn't prevent bind on top of another later on. To fix it
right way we need to mark _dentry_ as unsuitable for mounting
upon; new flag (DCACHE_CANT_MOUNT) is protected by d_flags and
i_mutex on the inode in question. Set it (with dont_mount(dentry))
in unlink/rmdir/etc., check (with cant_mount(dentry)) in places
in namespace.c that used to check for S_DEAD. Setting S_DEAD
is still needed in places where we used to set it (for directories
getting killed), since we rely on it for readdir/rmdir race
prevention.

2) rename()/mount() protection has another bogosity - we unhash
the target before we'd checked that it's not a mountpoint. Fixed.

3) ancient bogosity in pivot_root() - we locked i_mutex on the
right directory, but checked S_DEAD on the different (and wrong)
one. Noticed and fixed.

Signed-off-by: Al Viro <[e-mail]>

drivers/usb/core/inode.c

···
 	mutex_lock(&inode->i_mutex);
 	dentry_unhash(dentry);
 	if (usbfs_empty(dentry)) {
+		dont_mount(dentry);
 		drop_nlink(dentry->d_inode);
 		drop_nlink(dentry->d_inode);
 		dput(dentry);

fs/configfs/dir.c

···
 
 		configfs_detach_group(sd->s_element);
 		child->d_inode->i_flags |= S_DEAD;
+		dont_mount(child);
 
 		mutex_unlock(&child->d_inode->i_mutex);
 
···
 			mutex_lock(&dentry->d_inode->i_mutex);
 			configfs_remove_dir(item);
 			dentry->d_inode->i_flags |= S_DEAD;
+			dont_mount(dentry);
 			mutex_unlock(&dentry->d_inode->i_mutex);
 			d_delete(dentry);
 		}
···
 		if (ret) {
 			configfs_detach_item(item);
 			dentry->d_inode->i_flags |= S_DEAD;
+			dont_mount(dentry);
 		}
 		configfs_adjust_dir_dirent_depth_after_populate(sd);
 		mutex_unlock(&dentry->d_inode->i_mutex);
···
 	mutex_unlock(&configfs_symlink_mutex);
 	configfs_detach_group(&group->cg_item);
 	dentry->d_inode->i_flags |= S_DEAD;
+	dont_mount(dentry);
 	mutex_unlock(&dentry->d_inode->i_mutex);
 
 	d_delete(dentry);

fs/namei.c

···
 		error = security_inode_rmdir(dir, dentry);
 		if (!error) {
 			error = dir->i_op->rmdir(dir, dentry);
-			if (!error)
+			if (!error) {
 				dentry->d_inode->i_flags |= S_DEAD;
+				dont_mount(dentry);
+			}
 		}
 	}
 	mutex_unlock(&dentry->d_inode->i_mutex);
···
 		if (!error) {
 			error = dir->i_op->unlink(dir, dentry);
 			if (!error)
-				dentry->d_inode->i_flags |= S_DEAD;
+				dont_mount(dentry);
 		}
 	}
 	mutex_unlock(&dentry->d_inode->i_mutex);
···
 		return error;
 
 	target = new_dentry->d_inode;
-	if (target) {
+	if (target)
 		mutex_lock(&target->i_mutex);
-		dentry_unhash(new_dentry);
-	}
 	if (d_mountpoint(old_dentry)||d_mountpoint(new_dentry))
 		error = -EBUSY;
-	else 
+	else {
+		if (target)
+			dentry_unhash(new_dentry);
 		error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
+	}
 	if (target) {
-		if (!error)
+		if (!error) {
 			target->i_flags |= S_DEAD;
+			dont_mount(new_dentry);
+		}
 		mutex_unlock(&target->i_mutex);
 		if (d_unhashed(new_dentry))
 			d_rehash(new_dentry);
···
 		error = old_dir->i_op->rename(old_dir, old_dentry, new_dir, new_dentry);
 	if (!error) {
 		if (target)
-			target->i_flags |= S_DEAD;
+			dont_mount(new_dentry);
 		if (!(old_dir->i_sb->s_type->fs_flags & FS_RENAME_DOES_D_MOVE))
 			d_move(old_dentry, new_dentry);
 	}

fs/namespace.c

···
 
 	err = -ENOENT;
 	mutex_lock(&path->dentry->d_inode->i_mutex);
-	if (IS_DEADDIR(path->dentry->d_inode))
+	if (cant_mount(path->dentry))
 		goto out_unlock;
 
 	err = security_sb_check_sb(mnt, path);
···
 
 	err = -ENOENT;
 	mutex_lock(&path->dentry->d_inode->i_mutex);
-	if (IS_DEADDIR(path->dentry->d_inode))
+	if (cant_mount(path->dentry))
 		goto out1;
 
 	if (d_unlinked(path->dentry))
···
 	if (!check_mnt(root.mnt))
 		goto out2;
 	error = -ENOENT;
-	if (IS_DEADDIR(new.dentry->d_inode))
+	if (cant_mount(old.dentry))
 		goto out2;
 	if (d_unlinked(new.dentry))
 		goto out2;

include/linux/dcache.h

···
 
 #define DCACHE_FSNOTIFY_PARENT_WATCHED	0x0080 /* Parent inode is watched by some fsnotify listener */
 
+#define DCACHE_CANT_MOUNT	0x0100
+
 extern spinlock_t dcache_lock;
 extern seqlock_t rename_lock;
 
···
 static inline int d_unlinked(struct dentry *dentry)
 {
 	return d_unhashed(dentry) && !IS_ROOT(dentry);
+}
+
+static inline int cant_mount(struct dentry *dentry)
+{
+	return (dentry->d_flags & DCACHE_CANT_MOUNT);
+}
+
+static inline void dont_mount(struct dentry *dentry)
+{
+	spin_lock(&dentry->d_lock);
+	dentry->d_flags |= DCACHE_CANT_MOUNT;
+	spin_unlock(&dentry->d_lock);
 }
 
 static inline struct dentry *dget_parent(struct dentry *dentry)