Subject: PROBLEM: BUG appearing when trying to allocate interrupt on Exynos MCT after CPU hotplug

From: [email-masked] (Marcin Jabrzyk)
Date: Wed, 29 Oct 2014 11:38:09 +0100

So I've tried this patch, it resolves one problem but introduces also 
new ones. As expected the BUG warning is not showing after applying this 
patch but there are some interesting side effects.
I was looking on /proc/interrupts output. IRQ for CPU0 have "MCT" name 
and IRQ for CPU1 has unexpectedly no name at all.
After making hotplug cycle of CPU1 I've observed that IRQs attached 
originally for that CPU are generating on really low count and not in 
order with IRQ for CPU0.
What's more the interrupt for CPU1 is showing to me as being counted for 
both CPUs, so it's probably not being attached to CPU1.

Best regards,
Marcin Jabrzyk

On 27/10/14 21:16, Stephen Boyd wrote:
> On 10/24/2014 06:22 AM, Marcin Jabrzyk wrote:
>>
>>
>> On 23/10/14 20:41, Stephen Boyd wrote:
>>> On 10/23/2014 07:06 AM, Russell King - ARM Linux wrote:
>>>> The CPU notifier is called via notify_cpu_starting(), which is called
>>>> with interrupts disabled, and a reason code of CPU_STARTING.
>>>> Interrupts
>>>> at this point /must/ remain disabled.
>>>>
>>>> The Exynos code then goes on to call exynos4_local_timer_setup() which
>>>> tries to reverse the free_irq() in exynos4_local_timer_stop() by
>>>> calling
>>>> request_irq().  Calling request_irq() with interrupts off has never
>>>> been
>>>> permissible.
>>>>
>>>> So, this code is wrong today, and it was also wrong when it was
>>>> written.
>>>> It /couldn't/ have been tested.  It looks like this commit added this
>>>> buggy code:
>>>>
>>>> commit ee98d27df6827b5ba4bd99cb7d5cb1239b6a1a31
>>>> Author: Stephen Boyd <[email-masked]>
>>>> Date:   Fri Feb 15 16:40:51 2013 -0800
>>>>
>>>>       ARM: EXYNOS4: Divorce mct from local timer API
>>>>
>>>>       Separate the mct local timers from the local timer API. This will
>>>>       allow us to remove ARM local timer support in the near future and
>>>>       gets us closer to moving this driver to drivers/clocksource.
>>>>
>>>>       Acked-by: Kukjin Kim <[email-masked]>
>>>>       Acked-by: Marc Zyngier <[email-masked]>
>>>>       Cc: Thomas Abraham <[email-masked]>
>>>>       Signed-off-by: Stephen Boyd <[email-masked]>
>>>
>>> I'm not so sure. It looks like in that patch I didn't change anything
>>> with respect to when things are called. In fact, it looks like we were
>>> calling setup_irq() there, but another patch around the same time
>>> changed that to request_irq()
>>>
>>> commit 7114cd749a12ff9fd64a2f6f04919760f45ab183
>>> Author: Chander Kashyap <[email-masked]>
>>> Date:   Wed Jun 19 00:29:35 2013 +0900
>>>
>>>       clocksource: exynos_mct: use (request/free)_irq calls for local
>>> timer registration
>>>
>>>       Replace the (setup/remove)_irq calls for local timer
>>> registration with
>>>       (request/free)_irq calls. This generalizes the local timer
>>> registration API.
>>>       Suggested by Mark Rutland.
>>>
>>>       Signed-off-by: Chander Kashyap <[email-masked]>
>>>       Acked-by: Mark Rutland <[email-masked]>
>>>       Reviewed-by: Tomasz Figa <[email-masked]>
>>>       Signed-off-by: Kukjin Kim <[email-masked]>
>>>
>>> I don't believe setup_irq() allocates anything so we should probably go
>>> back to using that over request_irq() or explore requesting the irqs
>>> once and then enabling/disabling instead.
>>>
>>
>> So what would be a better way to handle this? Going back to setup_irq
>> or trying to enable/disable irqs on CPU hotplug? As this touched low
>> level things and it's rare case for setting/enabling irqs just after
>> CPU is coming back to life again.
>>
>
> The safest thing is setup_irq(), but do you care to try this patch?
> Doing the enable/disable is not as robust because request_irq() returns
> with the irq enabled and then we have to disable the irq to make things
> symmetric. This whole driver doesn't look like it's prepared for such a
> situation where the interrupt triggers before the clockevent is
> registered so this doesn't look like a problem in practice. Doing the
> disable right after request is typically bad though, and may not pass
> review.
>
> ----8<-----
>
> From: Stephen Boyd <[email-masked]>
> Subject: [PATCH] clocksource: exynos_mct: Avoid scheduling while atomic
>
> If we call request_irq() during the CPU_STARTING notifier we'll
> try to allocate an irq descriptor with GFP_KERNEL while we're
> running with irqs disabled. Just request the irqs at boot time
> and enable/disable them when a CPU comes up or goes down to avoid
> such problems.
>
> Signed-off-by: Stephen Boyd <[email-masked]>
> ---
>   drivers/clocksource/exynos_mct.c | 23 +++++++++++++----------
>   1 file changed, 13 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c
> index 9403061a2acc..1800053b4644 100644
> --- a/drivers/clocksource/exynos_mct.c
> +++ b/drivers/clocksource/exynos_mct.c
> @@ -467,13 +467,7 @@ static int exynos4_local_timer_setup(struct clock_event_device *evt)
>
>   	if (mct_int_type == MCT_INT_SPI) {
>   		evt->irq = mct_irqs[MCT_L0_IRQ + cpu];
> -		if (request_irq(evt->irq, exynos4_mct_tick_isr,
> -				IRQF_TIMER | IRQF_NOBALANCING,
> -				evt->name, mevt)) {
> -			pr_err("exynos-mct: cannot register IRQ %d\n",
> -				evt->irq);
> -			return -EIO;
> -		}
> +		enable_irq(evt->irq);
>   		irq_force_affinity(mct_irqs[MCT_L0_IRQ + cpu], cpumask_of(cpu));
>   	} else {
>   		enable_percpu_irq(mct_irqs[MCT_L0_IRQ], 0);
> @@ -488,7 +482,7 @@ static void exynos4_local_timer_stop(struct clock_event_device *evt)
>   {
>   	evt->set_mode(CLOCK_EVT_MODE_UNUSED, evt);
>   	if (mct_int_type == MCT_INT_SPI)
> -		free_irq(evt->irq, this_cpu_ptr(&percpu_mct_tick));
> +		disable_irq(evt->irq);
>   	else
>   		disable_percpu_irq(mct_irqs[MCT_L0_IRQ]);
>   }
> @@ -522,8 +516,9 @@ static struct notifier_block exynos4_mct_cpu_nb = {
>
>   static void __init exynos4_timer_resources(struct device_node *np, void __iomem *base)
>   {
> -	int err;
> +	int err, cpu;
>   	struct mct_clock_event_device *mevt = this_cpu_ptr(&percpu_mct_tick);
> +	struct mct_clock_event_device *evt;
>   	struct clk *mct_clk, *tick_clk;
>
>   	tick_clk = np ? of_clk_get_by_name(np, "fin_pll") :
> @@ -549,7 +544,15 @@ static void __init exynos4_timer_resources(struct device_node *np, void __iomem
>   		WARN(err, "MCT: can't request IRQ %d (%d)\n",
>   		     mct_irqs[MCT_L0_IRQ], err);
>   	} else {
> -		irq_set_affinity(mct_irqs[MCT_L0_IRQ], cpumask_of(0));
> +		for_each_present_cpu(cpu) {
> +			evt = per_cpu_ptr(&percpu_mct_tick, cpu);
> +			if (request_irq(mct_irqs[MCT_L0_IRQ + cpu],
> +					exynos4_mct_tick_isr,
> +					IRQF_TIMER | IRQF_NOBALANCING,
> +					"MCT", evt))
> +				pr_err("exynos-mct: cannot register IRQ\n");
> +			disable_irq(mct_irqs[MCT_L0_IRQ + cpu]);
> +		}
>   	}
>
>   	err = register_cpu_notifier(&exynos4_mct_cpu_nb);
>
--
Marcin Jabrzyk
Samsung R&D Institute Poland
Samsung Electronics