[PATCH] fs: Work around NFS wreckage

[Thomas Gleixner <tglx@linutronix.de>]

The dcache scalability work broke NFS root filesystems.

"cd /" results in the following problem:

    link_path_walk("/",...);
	jumps to return_reval
	need_reval_dot() returns true for NFS
	d_revalidate()
		dentry->d_op->d_revalidate(dentry, nd);
		 	returns -ECHILD due to nd->flags & LOOKUP_RCU
		nameidata_dentry_drop_rcu()
			spin_lock(&parent->d_lock);
			spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);

			This deadlocks because dentry == parent

This problem exists for any filesystem which implements d_revalidate.

Uwe bisected is down to commit 34286d6(fs: rcu-walk aware d_revalidate
method), but reverting that patch causes different wreckage to show up.

Check for parent equal dentry and skip the nested lock to avoid the
deadlock. I'm sure this is the wrong fix, but at least it "works" :)

Reported-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Reported-by: "Ramirez Luna, Omar" <omar.ramirez@ti.com>
Not-Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 fs/namei.c |    4 ++++
 1 file changed, 4 insertions(+)

Index: linux-2.6/fs/namei.c
===================================================================
--- linux-2.6.orig/fs/namei.c
+++ linux-2.6/fs/namei.c
@@ -487,6 +487,8 @@ static int nameidata_dentry_drop_rcu(str
 			goto err_root;
 	}
 	spin_lock(&parent->d_lock);
+	if (parent == dentry)
+		goto same;
 	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 	if (!__d_rcu_to_refcount(dentry, nd->seq))
 		goto err;
@@ -499,6 +501,8 @@ static int nameidata_dentry_drop_rcu(str
 	BUG_ON(!parent->d_count);
 	parent->d_count++;
 	spin_unlock(&dentry->d_lock);
+
+same:
 	spin_unlock(&parent->d_lock);
 	if (nd->root.mnt) {
 		path_get(&nd->root);