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Subject: Re: [PATCH] genirq: Remove warning on preemptible in
 prepare_percpu_nmi()
To:     Lecopzer Chen <lecopzer.chen@mediatek.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-arm-kernel@lists.infradead.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Julien Thierry <julien.thierry@arm.com>,
        YJ Chiang <yj.chiang@mediatek.com>
References: <20190620091233.22731-1-lecopzer.chen@mediatek.com>
From:   Marc Zyngier <marc.zyngier@arm.com>
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Organization: ARM Ltd
Message-ID: <7aebef0d-5795-0464-5dc8-6a2eb05fcc4e@arm.com>
Date:   Thu, 20 Jun 2019 10:40:39 +0100
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In-Reply-To: <20190620091233.22731-1-lecopzer.chen@mediatek.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Lecopzer,

On 20/06/2019 10:12, Lecopzer Chen wrote:
> prepare_percpu_nmi() acquires lock first by irq_get_desc_lock(),
> no matter whether preempt enabled or not, acquiring lock forces preempt off.
> 
> This simplifies the usage of prepare_percpu_nmi() and we don't need to
> acquire extra lock or explicitly call preempt_[disable,enable]().

I strongly disagree. If you're calling these functions *from* a
preemptible context, you've already lost, and that's what these
WARN_ON() calls are warning you about.

These functions can only be called from a context that is naturally
preemption free, such as a hotplug notifier. Otherwise, you have no idea
which CPU you're configuring the NMI on, and I cannot see that as a good
thing.

Thanks,

	M.

> 
> Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Marc Zyngier <marc.zyngier@arm.com>
> Cc: Julien Thierry <julien.thierry@arm.com>
> Cc: YJ Chiang <yj.chiang@mediatek.com>
> Cc: Lecopzer Chen <lecopzer.chen@mediatek.com>
> ---
>  kernel/irq/manage.c | 10 ----------
>  1 file changed, 10 deletions(-)
> 
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 78f3ddeb7fe4..aa03640cd7fb 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -2509,9 +2509,6 @@ int request_percpu_nmi(unsigned int irq, irq_handler_t handler,
>   *	This call prepares an interrupt line to deliver NMI on the current CPU,
>   *	before that interrupt line gets enabled with enable_percpu_nmi().
>   *
> - *	As a CPU local operation, this should be called from non-preemptible
> - *	context.
> - *
>   *	If the interrupt line cannot be used to deliver NMIs, function
>   *	will fail returning a negative value.
>   */
> @@ -2521,8 +2518,6 @@ int prepare_percpu_nmi(unsigned int irq)
>  	struct irq_desc *desc;
>  	int ret = 0;
>  
> -	WARN_ON(preemptible());
> -
>  	desc = irq_get_desc_lock(irq, &flags,
>  				 IRQ_GET_DESC_CHECK_PERCPU);
>  	if (!desc)
> @@ -2554,17 +2549,12 @@ int prepare_percpu_nmi(unsigned int irq)
>   *	This call undoes the setup done by prepare_percpu_nmi().
>   *
>   *	IRQ line should not be enabled for the current CPU.
> - *
> - *	As a CPU local operation, this should be called from non-preemptible
> - *	context.
>   */
>  void teardown_percpu_nmi(unsigned int irq)
>  {
>  	unsigned long flags;
>  	struct irq_desc *desc;
>  
> -	WARN_ON(preemptible());
> -
>  	desc = irq_get_desc_lock(irq, &flags,
>  				 IRQ_GET_DESC_CHECK_PERCPU);
>  	if (!desc)
> 


-- 
Jazz is not dead. It just smells funny...