From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 7 Mar 2012 21:00:34 +0100
Subject: [PATCH 187/353] fs: dcache: Use cpu_chill() in trylock loops
Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=54fa302171c7878902ca36c506e0939142c4905d

Retry loops on RT might loop forever when the modifying side was
preempted. Use cpu_chill() instead of cpu_relax() to let the system
make progress.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 fs/autofs/expire.c | 3 ++-
 fs/namespace.c     | 8 ++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/autofs/expire.c b/fs/autofs/expire.c
index 70e9afe589fb..1a6b88ad4fe0 100644
--- a/fs/autofs/expire.c
+++ b/fs/autofs/expire.c
@@ -8,6 +8,7 @@
  * option, any later version, incorporated herein by reference.
  */
 
+#include <linux/delay.h>
 #include "autofs_i.h"
 
 /* Check if a dentry can be expired */
@@ -153,7 +154,7 @@ static struct dentry *get_next_positive_dentry(struct dentry *prev,
 			parent = p->d_parent;
 			if (!spin_trylock(&parent->d_lock)) {
 				spin_unlock(&p->d_lock);
-				cpu_relax();
+				cpu_chill();
 				goto relock;
 			}
 			spin_unlock(&p->d_lock);
diff --git a/fs/namespace.c b/fs/namespace.c
index 396ff1bcfdad..2c0ff275e978 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -14,6 +14,7 @@
 #include <linux/mnt_namespace.h>
 #include <linux/user_namespace.h>
 #include <linux/namei.h>
+#include <linux/delay.h>
 #include <linux/security.h>
 #include <linux/cred.h>
 #include <linux/idr.h>
@@ -327,8 +328,11 @@ int __mnt_want_write(struct vfsmount *m)
 	 * incremented count after it has set MNT_WRITE_HOLD.
 	 */
 	smp_mb();
-	while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD)
-		cpu_relax();
+	while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD) {
+		preempt_enable();
+		cpu_chill();
+		preempt_disable();
+	}
 	/*
 	 * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
 	 * be set to match its requirements. So we must not load that until