linux-kernel - Re: [PATCH v2 17/76] ARC: Process-creation/scheduling/idle-loop

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Message-Id: <201301181435.15491.arnd@arndb.de>
Date:	Fri, 18 Jan 2013 14:35:15 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Frederic Weisbecker" <fweisbec@...il.com>
Subject: Re: [PATCH v2 17/76] ARC: Process-creation/scheduling/idle-loop

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
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