Subject: stomp_machine: Use mutex_trylock when called from inactive cpu
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 03 Oct 2012 17:21:53 +0100

If the stop machinery is called from inactive CPU we cannot use
mutex_lock, because some other stomp machine invokation might be in
progress and the mutex can be contended. We cannot schedule from this
context, so trylock and loop.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
---
 kernel/stop_machine.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

Index: linux-stable/kernel/stop_machine.c
===================================================================
--- linux-stable.orig/kernel/stop_machine.c
+++ linux-stable/kernel/stop_machine.c
@@ -158,7 +158,7 @@ static DEFINE_PER_CPU(struct cpu_stop_wo
 
 static void queue_stop_cpus_work(const struct cpumask *cpumask,
 				 cpu_stop_fn_t fn, void *arg,
-				 struct cpu_stop_done *done)
+				 struct cpu_stop_done *done, bool inactive)
 {
 	struct cpu_stop_work *work;
 	unsigned int cpu;
@@ -175,7 +175,12 @@ static void queue_stop_cpus_work(const s
 	 * Make sure that all work is queued on all cpus before we
 	 * any of the cpus can execute it.
 	 */
-	mutex_lock(&stopper_lock);
+	if (!inactive) {
+		mutex_lock(&stopper_lock);
+	} else {
+		while (!mutex_trylock(&stopper_lock))
+			cpu_relax();
+	}
 	for_each_cpu(cpu, cpumask)
 		cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
 				    &per_cpu(stop_cpus_work, cpu));
@@ -188,7 +193,7 @@ static int __stop_cpus(const struct cpum
 	struct cpu_stop_done done;
 
 	cpu_stop_init_done(&done, cpumask_weight(cpumask));
-	queue_stop_cpus_work(cpumask, fn, arg, &done);
+	queue_stop_cpus_work(cpumask, fn, arg, &done, false);
 	wait_for_stop_done(&done);
 	return done.executed ? done.ret : -ENOENT;
 }
@@ -601,7 +606,7 @@ int stop_machine_from_inactive_cpu(int (
 	set_state(&smdata, STOPMACHINE_PREPARE);
 	cpu_stop_init_done(&done, num_active_cpus());
 	queue_stop_cpus_work(cpu_active_mask, stop_machine_cpu_stop, &smdata,
-			     &done);
+			     &done, true);
 	ret = stop_machine_cpu_stop(&smdata);
 
 	/* Busy wait for completion. */