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 <b...@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 <b...@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 <b...@kernel.crashing.org>
Signed-off-by: Peter Zijlstra <a.p.zijls...@chello.nl>
LKML-Reference: <1271212509.13059.135.ca...@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
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