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From: Arnd Bergmann <arnd@arndb.de>
To: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Subject: Re: [PATCH v2 17/76] ARC: Process-creation/scheduling/idle-loop
Date: Fri, 18 Jan 2013 14:35:15 +0000
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        Al Viro <viro@zeniv.linux.org.uk>,
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On Friday 18 January 2013, Vineet Gupta wrote:
> +void cpu_idle(void)
> +{
> +       /* Since we SLEEP in idle loop, TIF_POLLING_NRFLAG can't be set */
> +
> +       /* endless idle loop with no priority at all */
> +       while (1) {
> +               tick_nohz_idle_enter();
> +               rcu_idle_enter();
> +
> +               while (!need_resched())
> +                       arch_idle();
> +
> +               rcu_idle_exit();
> +               tick_nohz_idle_exit();
> +
> +               schedule_preempt_disabled();
> +       }
> +}

Unless I'm mistaken, you have introduced the classic sleep race
here, where an interrupt can happen between the check for
need_resched() and the sleep instruction in arch_idle().

To avoid that, you need to disable interrupts around
the inner loop. The sleep instruction should return with
interrupts implicitly enabled if ARC behaves like most
other architectures doing this.

	Arnd