From: Thomas Gleixner <tglx@linutronix.de>
Date: Thu, 20 Dec 2012 18:28:26 +0100
Subject: block: Use cpu_chill() for retry loops
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git
Git-commit: 0ccd3ffff31c1aebb2b7ea4331037ccde76e043d
Patch-mainline: Queued in subsystem maintainer repository
References: SLE Realtime Extension

Retry loops on RT might loop forever when the modifying side was
preempted. Steven also observed a live lock when there was a
concurrent priority boosting going on.

Use cpu_chill() instead of cpu_relax() to let the system
make progress.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Mike Galbraith <mgalbraith@suse.de>
---
 block/blk-ioc.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -8,6 +8,7 @@
 #include <linux/blkdev.h>
 #include <linux/slab.h>
 #include <linux/sched/task.h>
+#include <linux/delay.h>
 
 #include "blk.h"
 
@@ -117,7 +118,7 @@ static void ioc_release_fn(struct work_s
 			spin_unlock(q->queue_lock);
 		} else {
 			spin_unlock_irqrestore(&ioc->lock, flags);
-			cpu_relax();
+			cpu_chill();
 			spin_lock_irqsave_nested(&ioc->lock, flags, 1);
 		}
 	}
@@ -201,7 +202,7 @@ void put_io_context_active(struct io_con
 				spin_unlock(icq->q->queue_lock);
 			} else {
 				spin_unlock_irqrestore(&ioc->lock, flags);
-				cpu_relax();
+				cpu_chill();
 				goto retry;
 			}
 		}