Re: workqueue: WARN at at kernel/workqueue.c:2176

[Peter Zijlstra]

On Fri, May 16, 2014 at 11:35:30AM +0200, Peter Zijlstra wrote:
> On Fri, May 16, 2014 at 11:50:42AM +0800, Lai Jiangshan wrote:
> > After debugging, I found the hotlug-in cpu is atctive but !online in this 
> > case.
> > the problem was introduced by 5fbd036b.
> > Some code assumes that any cpu in cpu_active_mask is also online, but 
> > 5fbd036b breaks
> > this assumption, so the corresponding code with this assumption should be 
> > changed too.
> 
> Good find, and yes it does that.
> 
> > The following patch is just a workaround. After it is applied, the above 
> > WARNING
> > is gone, but I can't hit the wq problem that you found.
> 
> Seeing how the entirety of hotplug is basically duct tape and twigs, the
> below isn't that bad.


I made that, are you okay with that?

---
Subject: sched: Fix hotplug vs set_cpus_allowed_ptr()
From: Lai Jiangshan <la...@cn.fujitsu.com>
Date: Fri, 16 May 2014 11:50:42 +0800

Lai found that:

  WARNING: CPU: 1 PID: 13 at arch/x86/kernel/smp.c:124 
native_smp_send_reschedule+0x2d/0x4b()
  ...
  migration_cpu_stop+0x1d/0x22

was caused by set_cpus_allowed_ptr() assuming that cpu_active_mask is
always a sub-set of cpu_online_mask.

This isn't true since 5fbd036b552f ("sched: Cleanup cpu_active
madness").

So set active and online at the same time to avoid this particular
problem.

Fixes: 5fbd036b552f ("sched: Cleanup cpu_active madness")
Signed-off-by: Lai Jiangshan <la...@cn.fujitsu.com>
Signed-off-by: Peter Zijlstra <pet...@infradead.org>
Link: http://lkml.kernel.org/r/53758b12.8060...@cn.fujitsu.com
---
 kernel/cpu.c        |    6 ++++--
 kernel/sched/core.c |    1 -
 2 files changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -726,10 +726,12 @@ void set_cpu_present(unsigned int cpu, b
 
 void set_cpu_online(unsigned int cpu, bool online)
 {
-       if (online)
+       if (online) {
                cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
-       else
+               cpumask_set_cpu(cpu, to_cpumask(cpu_active_bits));
+       } else {
                cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
+       }
 }
 
 void set_cpu_active(unsigned int cpu, bool active)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5126,7 +5126,6 @@ static int sched_cpu_active(struct notif
                                      unsigned long action, void *hcpu)
 {
        switch (action & ~CPU_TASKS_FROZEN) {
-       case CPU_STARTING:
        case CPU_DOWN_FAILED:
                set_cpu_active((long)hcpu, true);
                return NOTIFY_OK;

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