Possible bug with mutex adaptative spinning

[Peter Zijlstra]

On Wed, 2010-04-14 at 12:35 +1000, Benjamin Herrenschmidt wrote:

> [PATCH] mutex: Don't spin when the owner CPU is offline or other weird cases
> 
> The current code might spin forever if the CPU owning the mutex has been
> offlined, and the last CPU in the system is trying to acquire it, since
> mutex_spin_on_owner() will always return 1, telling the caller to spin
> until the mutex has been released.
> 
> This patch changes mutex_spin_on_owner() to return 0 (don't spin) in any
> case where we aren't sure about the owner struct validity or CPU number,
> and if the said CPU is offline. There is no point going back &
> re-evaluate spinning in corner cases like that, let's just go to sleep.
> 
> Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>

Right, patch looks good, comments are a tad misleading.


I've queued the below patch

---
Subject: mutex: Don't spin when the owner CPU is offline or other weird cases
From: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Date: Fri Apr 16 23:20:00 CEST 2010

Due to recent load-balancer changes that delay the task migration to
the next wakeup, the adaptive mutex spinning ends up in a live lock
when the owner's CPU gets offlined because the cpu_online() check
lives before the owner running check.

This patch changes mutex_spin_on_owner() to return 0 (don't spin) in
any case where we aren't sure about the owner struct validity or CPU
number, and if the said CPU is offline. There is no point going back &
re-evaluate spinning in corner cases like that, let's just go to
sleep.

Signed-off-by: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra at chello.nl>
LKML-Reference: <1271212509.13059.135.camel at pasglop>
---
 kernel/sched.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -3647,7 +3647,7 @@ int mutex_spin_on_owner(struct mutex *lo
 	 * the mutex owner just released it and exited.
 	 */
 	if (probe_kernel_address(&owner->cpu, cpu))
-		goto out;
+		return 0;
 #else
 	cpu = owner->cpu;
 #endif
@@ -3657,14 +3657,14 @@ int mutex_spin_on_owner(struct mutex *lo
 	 * the cpu field may no longer be valid.
 	 */
 	if (cpu >= nr_cpumask_bits)
-		goto out;
+		return 0;
 
 	/*
 	 * We need to validate that we can do a
 	 * get_cpu() and that we have the percpu area.
 	 */
 	if (!cpu_online(cpu))
-		goto out;
+		return 0;
 
 	rq = cpu_rq(cpu);
 
@@ -3683,7 +3683,7 @@ int mutex_spin_on_owner(struct mutex *lo
 
 		cpu_relax();
 	}
-out:
+
 	return 1;
 }
 #endif