linux-kernel - Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi

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Date:   Thu, 21 Apr 2022 13:42:59 +0200
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Pingfan Liu <kernelfans@...il.com>
Cc:     linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse()
 in show_msi_interrupt()

On Thu, Apr 21, 2022 at 11:36:17AM +0800, Pingfan Liu wrote:
> Oops. You are right. What about using rcu_read_lock() directly?
> 
> 
> diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
> index 3033f616e256..45393919fe61 100644
> --- a/arch/s390/kernel/irq.c
> +++ b/arch/s390/kernel/irq.c
> @@ -205,7 +205,7 @@ static void show_msi_interrupt(struct seq_file *p, int irq)
>  	unsigned long flags;
>  	int cpu;
>  
> -	irq_lock_sparse();
> +	rcu_read_lock();
>  	desc = irq_to_desc(irq);
>  	if (!desc)
>  		goto out;
> @@ -224,7 +224,7 @@ static void show_msi_interrupt(struct seq_file *p, int irq)
>  	seq_putc(p, '\n');
>  	raw_spin_unlock_irqrestore(&desc->lock, flags);
>  out:
> -	irq_unlock_sparse();
> +	rcu_read_unlock();

That looks like it should work. Please resend and also add a reference
to commit 74bdf7815dfb ("genirq: Speedup show_interrupts()") which
explains why this works.