configfs: Fix race between configfs_readdir() and configfs_d_iput()

configfs_readdir() will use the existing inode numbers of inodes in the
dcache, but it makes them up for attribute files that aren't currently
instantiated.  There is a race where a closing attribute file can be
tearing down at the same time as configfs_readdir() is trying to get its
inode number.

We want to get the inode number of open attribute files, because they
should match while instantiated.  We can't lock down the transition
where dentry->d_inode is set to NULL, so we just check for NULL there.
We can, however, ensure that an inode we find isn't iput() in
configfs_d_iput() until after we've accessed it.
Signed-off-by: Joel Becker <jlbec@evilplan.org>

fs/configfs/dir.c

@@ -53,11 +53,14 @@ DEFINE_SPINLOCK(configfs_dirent_lock);
 static void configfs_d_iput(struct dentry * dentry,
 			    struct inode * inode)
 {
-	struct configfs_dirent * sd = dentry->d_fsdata;
+	struct configfs_dirent *sd = dentry->d_fsdata;
 
 	if (sd) {
 		BUG_ON(sd->s_dentry != dentry);
+		/* Coordinate with configfs_readdir */
+		spin_lock(&configfs_dirent_lock);
 		sd->s_dentry = NULL;
+		spin_unlock(&configfs_dirent_lock);
 		configfs_put(sd);
 	}
 	iput(inode);
@@ -1546,7 +1549,7 @@ static int configfs_readdir(struct file * filp, void * dirent, filldir_t filldir
 	struct configfs_dirent * parent_sd = dentry->d_fsdata;
 	struct configfs_dirent *cursor = filp->private_data;
 	struct list_head *p, *q = &cursor->s_sibling;
-	ino_t ino;
+	ino_t ino = 0;
 	int i = filp->f_pos;
 
 	switch (i) {
@@ -1574,6 +1577,7 @@ static int configfs_readdir(struct file * filp, void * dirent, filldir_t filldir
 				struct configfs_dirent *next;
 				const char * name;
 				int len;
+				struct inode *inode = NULL;
 
 				next = list_entry(p, struct configfs_dirent,
 						   s_sibling);
@@ -1582,9 +1586,28 @@ static int configfs_readdir(struct file * filp, void * dirent, filldir_t filldir
 
 				name = configfs_get_name(next);
 				len = strlen(name);
-				if (next->s_dentry)
-					ino = next->s_dentry->d_inode->i_ino;
-				else
+
+				/*
+				 * We'll have a dentry and an inode for
+				 * PINNED items and for open attribute
+				 * files.  We lock here to prevent a race
+				 * with configfs_d_iput() clearing
+				 * s_dentry before calling iput().
+				 *
+				 * Why do we go to the trouble?  If
+				 * someone has an attribute file open,
+				 * the inode number should match until
+				 * they close it.  Beyond that, we don't
+				 * care.
+				 */
+				spin_lock(&configfs_dirent_lock);
+				dentry = next->s_dentry;
+				if (dentry)
+					inode = dentry->d_inode;
+				if (inode)
+					ino = inode->i_ino;
+				spin_unlock(&configfs_dirent_lock);
+				if (!inode)
 					ino = iunique(configfs_sb, 2);
 
 				if (filldir(dirent, name, len, filp->f_pos, ino,