From: Ian Kent <ikent@redhat.com>
Date: Wed, 18 Mar 2009 16:05:32 +0900
Subject: [fs] autofs4: fix lookup deadlock
Message-id: 20090318070532.9081.50926.stgit@zeus.themaw.net
O-Subject: [RHEL 5.4 PATCH 5/5] autofs4 - fix lookup deadlock
Bugzilla: 490078

From: Ian Kent <raven@themaw.net>

This patch is needed to addresses bug 490078.

A deadlock can occur when user space uses a signal (ie. autofs version
4 uses SIGCHLD for this) to effect expire completion.

The order of events is:

Expire process completes, but before being able to send SIGCHLD to it's
parent ...

Another process walks onto a different mount point and drops the directory
inode semaphore prior to sending the request to the daemon as it must ...

A third process does an lstat on on the expired mount point causing it to
wait on expire completion (unfortunately) holding the directory semaphore.

The mount request then arrives at the daemon which does an lstat and,
deadlock.

For a long time I was concerned about releasing the directory semaphore
around the expire wait in autofs4_lookup, as well as for the mount call
back. I finally realised that the last round of changes in this function
made the expiring dentry and the lookup dentry separate and distinct so
the check and possible wait can be done anywhere prior to the mount call
back. This patch moves the check to just before the mount call back and
inside the directory inode mutex release.

diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index d474338..b08d1fa 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -482,22 +482,6 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 	DPRINTK("pid = %u, pgrp = %u, catatonic = %d, oz_mode = %d",
 		 current->pid, process_group(current), sbi->catatonic, oz_mode);
 
-	expiring = autofs4_lookup_expiring(sbi, dentry->d_parent, &dentry->d_name);
-	if (expiring) {
-		/*
-		 * If we are racing with expire the request might not
-		 * be quite complete but the directory has been removed
-		 * so it must have been successful, so just wait for it.
-		 */
-		ino = autofs4_dentry_ino(expiring);
-		autofs4_expire_wait(expiring);
-		spin_lock(&sbi->lookup_lock);
-		if (!list_empty(&ino->expiring))
-			list_del_init(&ino->expiring);
-		spin_unlock(&sbi->lookup_lock);
-		dput(expiring);
-	}
-
 	unhashed = autofs4_lookup_active(sbi, dentry->d_parent, &dentry->d_name);
 	if (unhashed)
 		dentry = unhashed;
@@ -535,14 +519,31 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 	}
 
 	if (!oz_mode) {
+		mutex_unlock(&dir->i_mutex);
+		expiring = autofs4_lookup_expiring(sbi,
+						   dentry->d_parent,
+						   &dentry->d_name);
+		if (expiring) {
+			/*
+			 * If we are racing with expire the request might not
+			 * be quite complete but the directory has been removed
+			 * so it must have been successful, so just wait for it.
+			 */
+			ino = autofs4_dentry_ino(expiring);
+			autofs4_expire_wait(expiring);
+			spin_lock(&sbi->lookup_lock);
+			if (!list_empty(&ino->expiring))
+				list_del_init(&ino->expiring);
+			spin_unlock(&sbi->lookup_lock);
+			dput(expiring);
+		}
+
 		spin_lock(&dentry->d_lock);
 		dentry->d_flags |= DCACHE_AUTOFS_PENDING;
 		spin_unlock(&dentry->d_lock);
-		if (dentry->d_op && dentry->d_op->d_revalidate) {
-			mutex_unlock(&dir->i_mutex);
+		if (dentry->d_op && dentry->d_op->d_revalidate)
 			(dentry->d_op->d_revalidate)(dentry, nd);
-			mutex_lock(&dir->i_mutex);
-		}
+		mutex_lock(&dir->i_mutex);
 	}
 
 	/*