[PATCH] autofs4: fix race in unhashed dentry code

Commit f50b6f8691cae2e0064c499dd3ef3f31142987f0 introduced a race in
autofs4 between autofs_lookup_unhashed() and autofs_dentry_release().

autofs_dentry_release() ends up clearing the ->dentry and ->inode members
of autofs_info before removing it from the rehash list.  The list is
protected by the rehash lock in both functions, but since
autofs_dentry_release() starts tearing the autofs_info struct down before
removing it from the list, autofs_lookup_unhashed() can get a autofs_info
with a NULL dentry.

This patch moves the clearing of ->dentry and ->inode after the removal
from the rehash list.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/root.c

@@ -470,9 +470,6 @@ void autofs4_dentry_release(struct dentry *de)
 	if (inf) {
 		struct autofs_sb_info *sbi = autofs4_sbi(de->d_sb);
 
-		inf->dentry = NULL;
-		inf->inode = NULL;
-
 		if (sbi) {
 			spin_lock(&sbi->rehash_lock);
 			if (!list_empty(&inf->rehash))
@@ -480,6 +477,9 @@ void autofs4_dentry_release(struct dentry *de)
 			spin_unlock(&sbi->rehash_lock);
 		}
 
+		inf->dentry = NULL;
+		inf->inode = NULL;
+
 		autofs4_free_ino(inf);
 	}
 }