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

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Message-ID: <20130514141306.GB19719@somewhere>
Date:	Tue, 14 May 2013 16:13:07 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	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 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.

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

Thanks!
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