linux-kernel - Re: next-20130627 breaks i.MX6 sabre sd UART console

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Message-ID: <alpine.DEB.2.02.1307020016580.4013@ionos.tec.linutronix.de>
Date:	Tue, 2 Jul 2013 00:22:25 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stephen Boyd <sboyd@...eaurora.org>
cc:	Stehle Vincent-B46079 <B46079@...escale.com>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Rutland <mark.rutland@....com>
Subject: Re: next-20130627 breaks i.MX6 sabre sd UART console

On Mon, 1 Jul 2013, Stephen Boyd wrote:
> On 07/01/13 14:24, Thomas Gleixner wrote:
> > On Mon, 1 Jul 2013, Stephen Boyd wrote:
> >> On 07/01/13 13:14, Thomas Gleixner wrote:
> >>> The issue is very subtle. What happens is:
> >>>
> >>> CPU0						CPU1
> >>>
> >>> Switch to oneshot mode
> >>>
> >>>  Copy the bits from tick_broadcast_mask to
> >>>  tick_broadcast_oneshot_mask. We need to do
> >>>  that so the other cpus reach the timer irq
> >>>  and the softirq which switches them to
> >>>  oneshot.
> >>>
> >>>  Kick the broadcast device into oneshot.
> >>>
> >>> 						Timer interrupt fires
> >>> 						
> >>> 						irq_enter sees the cpu in
> >>> 						tick_broadcast_oneshot_mask and
> >>> 						sets the device to oneshot mode
> >>> 						
> >>> 						handle_periodic:
> >>> 						 Sees oneshot mode and adds
> >>> 						 period to
> >>> 						 dev->next_event(KTIME_MAX)
> >>> 			
> >> Yep. It is also racing with the timer interrupt so having more than two
> >> CPUs must help widen the window (which is why we see it on the higher
> >> numbered CPUs).
> > The race above is about the timer interrupt. You mean the broadcast
> > one which is still enabled due to the dummy -> functional transition
> > issue, right? That helps a lot to make this more visible, because we
> > double the number of events.
> 
> I was thinking that tick_check_oneshot_broadcast() is racing with
> tick_switch_to_oneshot() and because we have more CPUs we're more likely
> to have a CPU fix up the handler in tick_switch_to_oneshot() after
> tick_check_oneshot_broadcast() forces that CPU to oneshot mode and the
> periodic handler runs. I wonder if I can reproduce it locally by making
> tick_switch_to_oneshot() spin for a jiffy or two on CPU1.

tick_switch_to_oneshot() is invoked with interrupts disabled, so that
wont help. 

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