fs-namespace-preemption-fix.patch

From 92b0ddb74ba34dba91878a346c3c6bf396ffb931 Mon Sep 17 00:00:00 2001
From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 19 Jul 2009 08:44:27 -0500
Subject: [PATCH] fs: namespace preemption fix

commit ce5c9d0cfb49aadeac6fe322035c4f73bbc5ef85 in tip.

On RT we cannot loop with preemption disabled here as
mnt_make_readonly() might have been preempted. Instead we block on
vfsmount_lock which is held by mnt_make_readonly(). Works for !RT as
well.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/fs/namespace.c b/fs/namespace.c
index c768f73..962fd96 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -264,8 +264,16 @@ int mnt_want_write(struct vfsmount *mnt)
 	 * incremented count after it has set MNT_WRITE_HOLD.
 	 */
 	smp_mb();
-	while (mnt->mnt_flags & MNT_WRITE_HOLD)
-		cpu_relax();
+	preempt_enable();
+	/*
+	 * HACK ALERT. on RT we can not spin here with cpu_relax() and
+	 * preemption disabled so we block on the vfsmount lock which is
+	 * held by mnt_make_readonly(). Works on !RT as well.
+	 */
+	while (mnt->mnt_flags & MNT_WRITE_HOLD) {
+		spin_lock(&vfsmount_lock);
+		spin_unlock(&vfsmount_lock);
+	}
 	/*
 	 * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
 	 * be set to match its requirements. So we must not load that until
@@ -273,12 +281,11 @@ int mnt_want_write(struct vfsmount *mnt)
 	 */
 	smp_rmb();
 	if (__mnt_is_readonly(mnt)) {
+		preempt_disable();
 		dec_mnt_writers(mnt);
+		preempt_enable();
 		ret = -EROFS;
-		goto out;
 	}
-out:
-	preempt_enable();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(mnt_want_write);
-- 
1.7.1.1