linux-kernel - Re: [PATCH] machine_power_off: not only local_irq

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Message-ID: <20140705192533.GS21766@n2100.arm.linux.org.uk>
Date:	Sat, 5 Jul 2014 20:25:33 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	pawandeep oza <oza.contri.linux.kernel@...il.com>
Cc:	akpm@...ux-foundation.org, holt@....com, mingo@...nel.org,
	sboyd@...eaurora.org, k.khlebnikov@...sung.com,
	u.kleine-koenig@...gutronix.de,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	linux-arm-kernel@...ts.infradead.or, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] machine_power_off: not only local_irq_disable but also
	do disable preemption

On Sun, Jul 06, 2014 at 12:20:03AM +0530, pawandeep oza wrote:
> preempt_check_resched would check TIF_NEED_RESCHED
> #define preempt_check_resched() \
> do { \
> if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
> preempt_schedule(); \
> } while (0)
> 
> there is a chance that just beofre we disabled irqs, somebody would have
> marked the flag to current, and
> later on, it might happen that, current gets replaced by the process which
> tries to hold a spin_lock which has already been previosuly held by CPU1
> when
> was being plugged out by smp_send_stop.

And preempt_schedule() contains:

        /*
         * If there is a non-zero preempt_count or interrupts are disabled,
         * we do not want to preempt the current task. Just return..
         */
        if (likely(!preemptible()))
                return;

where preemptible() is defined as:

	(preempt_count() == 0 && !irqs_disabled())

which means... if interrupts are disabled (as they are) we don't
return from preempt_schedule() without doing anything.

Scheduling with interrupts disabled is a bug.  If you need to add
preempt_disable() before local_irq_disable() to prevent it, then
there's a bug somewhere else, and we don't "fix" that by adding
preempt_disable().  The real bug needs to be found and fixed.

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