linux-kernel - Re: [PATCH -next 2/3] genirq/affinity: Define tmp_mask as a local variable in irq_do_set

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Date:   Sat, 27 Aug 2022 16:24:17 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Xu Qiang <xuqiang36@...wei.com>
Cc:     <tglx@...utronix.de>, <frederic@...nel.org>,
        <peterz@...radead.org>, <nitesh@...hat.com>,
        <bigeasy@...utronix.de>, <douliyangs@...il.com>,
        <linux-kernel@...r.kernel.org>, <guohanjun@...wei.com>,
        <weiyongjun1@...wei.com>
Subject: Re: [PATCH -next 2/3] genirq/affinity: Define tmp_mask as a local variable in irq_do_set_affinity

On Sat, 27 Aug 2022 02:13:50 +0100,
Xu Qiang <xuqiang36@...wei.com> wrote:
> 
> When irq_do_set_affinity is called, tmp_mask saved last time
> does not make any sense. it is reassigned before each use,
> so it should be defined as a local variable.
> 
> Fixes: 33de0aa4bae9 (“genirq: Always limit the affinity to online CPUs”)
> Signed-off-by: Xu Qiang <xuqiang36@...wei.com>
> ---
>  kernel/irq/manage.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index c3423f552e0b..ae1c7eebdfa6 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -218,7 +218,7 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
>  	int ret;
>  
>  	static DEFINE_RAW_SPINLOCK(tmp_mask_lock);
> -	static struct cpumask tmp_mask;
> +	struct cpumask tmp_mask;
>  
>  	if (!chip || !chip->irq_set_affinity)
>  		return -EINVAL;

Oh Gawd... Don't you see *WHY* this is a static structure? Hint: what
is the effect of this on a machine with 4096 CPUs when the stack space
is tight? And what would be the point of having a spinlock to protect
a local variable?

	M.


-- 
Without deviation from the norm, progress is not possible.