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

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Date:	Sat, 11 May 2013 03:36:30 +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);

So since it's only ever called right before resuming to userspace and after
the user_exit() call from the end of the syscall/exception/irq code,
you can use user_enter()/user_exit() directly.

I'm also wondering if this assumption that irqs are disabled by the time
we do user preemption is x86-centric or not. May be we can wait for complains
from those who'll port it...
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