linux-kernel - Re: [PATCH 1/2] sched/mm: add finish_switch

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Date:	Wed, 13 Nov 2013 17:03:58 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] sched/mm: add finish_switch_mm function

On Wed, Nov 13, 2013 at 04:05:56PM +0000, Martin Schwidefsky wrote:
> On Wed, 13 Nov 2013 12:19:09 +0000
> Catalin Marinas <catalin.marinas@....com> wrote:
> > On Wed, Nov 13, 2013 at 11:41:43AM +0000, Peter Zijlstra wrote:
> > > On Wed, Nov 13, 2013 at 09:16:13AM +0100, Martin Schwidefsky wrote:
> > > >  	fire_sched_in_preempt_notifiers(current);
> > > >  	if (mm)
> > > > @@ -4140,8 +4141,10 @@ void idle_task_exit(void)
> > > >  
> > > >  	BUG_ON(cpu_online(smp_processor_id()));
> > > >  
> > > > -	if (mm != &init_mm)
> > > > +	if (mm != &init_mm) {
> > > >  		switch_mm(mm, &init_mm, current);
> > > > +		finish_switch_mm(&init_mm, current);
> > > > +	}
> > > >  	mmdrop(mm);
> > > >  }
> > 
> > Here finish_switch_mm() is called in the same context with switch_mm().
> > What we have on ARM via switch_mm() is to check for irqs_disabled() and
> > if yes, defer the actual switching via a flag until the
> > finish_arch_post_lock_switch() hook. But on ARM we only cared about the
> > interrupts being enabled.
> 
> The guarantee s390 needs is that the rq-lock is not taken. What I have
> seen with the wait loop in switch_mm is a dead lock because one CPU #0
> was looping in switch_mm to wait for the TLB flush of another CPU #1.
> CPU #1 got an interrupt that tried to wake-up a task which happened to
> be on the run-queue of CPU #0.

I'm not familiar with the s390 code, so how's the waiting done? Is it
part of an on_each_cpu() call (that's what I got from smp_ptlb_all)?

-- 
Catalin
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