linux-kernel - Re: [PATCH 1/3] x86/sched/context_tracking: Call new schedule_preempt_user() from entry

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Date:	Wed, 15 May 2013 09:38:50 +0800
From:	Li Zhong <zhong@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] x86/sched/context_tracking: Call new
 schedule_preempt_user() from entry_64.S

On Tue, 2013-05-14 at 16:13 +0200, Frederic Weisbecker wrote:
> On Fri, May 10, 2013 at 05:12:26PM -0400, Steven Rostedt wrote:
> > +/*
> > + * This is a entry point to the scheduler() just before going
> > + * back to user space. This is called with irqs disabled
> > + * which prevents races with the CONTEXT_TRACKING updates.
> > + */
> > +asmlinkage void __sched schedule_preempt_user(void)
> > +{
> > +	enum ctx_state prev_state;
> > +
> > +	prev_state = exception_enter();
> > +
> > +	local_irq_enable();
> > +	__schedule();
> > +	local_irq_disable();
> > +
> > +	exception_exit(prev_state);
> > +}
> 
> Ok I just had a look at how ARM and PPC64 are handling user preemption and it seems
> that irqs are disabled around the call to schedule() on these archs too. Although
> do_work_pending() in ARM surprisingly doesn't re-enable irqs before calling schedule?
> 
> Anyway having irqs disabled around user preemption seem to be a requirement to make
> sure the TIF_NEED_RESCHED check is not racy against irqs and return to userspace.
> So I guess we can keep the above function as it is.
> 
> But perhaps we should queue this for 3.11 given that it's a bit of a sensitive change
> in the x86 user return path.
> 
> Look, I'm just going to make a seperate pull request with this patch based on 3.10-rc1
> and let Ingo choose the target.

Could the deletion of schedule_user() be separated to another patch. So
for other archs, such as ppc64, we could have both
schedule_preempt_user() and schedule_user() there for the conversion?

Or are there some better ways to avoid the conflict with arch trees? 

Thanks, Zhong

> 
> (Meanwhile I still think it would be a good idea to keep LOCKDEP_SYS_EXIT in the loop :-)
> 
> Thanks!
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