linux-kernel - Re: [PATCH 7/9] irq: remove needless lock in takedown

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Date:   Mon, 25 Apr 2022 10:57:05 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Baokun Li <libaokun1@...wei.com>,
        Mark Rutland <mark.rutland@....com>,
        Steven Price <steven.price@....com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Yuan ZhaoXiong <yuanzhaoxiong@...du.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH 7/9] irq: remove needless lock in takedown_cpu()

On Thu, Apr 21, 2022 at 06:11:56PM +0200, Thomas Gleixner wrote:
> On Wed, Apr 20 2022 at 22:05, Pingfan Liu wrote:
> 
> First of all, the subject prefix for the core interrupt subsystem is
> 'genirq' and the sentence after the colon starts with an uppercase
> letter. See:
> 
> https://www.kernel.org/doc/html/latest/process/maintainer-tip.html
> 
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index d0a9aa0b42e8..94a6b512c26d 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -1033,18 +1033,16 @@ static int takedown_cpu(unsigned int cpu)
> >  	kthread_park(st->thread);
> >  
> >  	/*
> > -	 * Prevent irq alloc/free while the dying cpu reorganizes the
> > -	 * interrupt affinities.
> > +	 * RCU keeps watching 'cpu' until do_idle()->rcu_report_dead().
> > +	 * And cpu_stopper's fn is dispatched with preemption disabled.
> > +	 * So it can not occur to release a irq_desc.
> >  	 */
> > -	irq_lock_sparse();
> 
> Not everything is about RCU here. You really need to look at all moving
> parts:
> 
> irq_migrate_all_off_this_cpu() relies on the allocated_irqs bitmap and
> the sparse tree to be in consistent state, which is only guaranteed when
> the sparse lock is held.
> 

For the irq which transfer from active to inactive(disappearing) after
fetching, desc->lock can serve the sync purpose. In this case,
irq_lock_sparse() is not needed. For a emergeing irq, I am not sure
about it.

> I'm not sure what you are trying to solve here. Not taking sparse_irq_lock
> here is not gaining anything.
>

It was a big lock preventing my original series to make kexec-reboot parallel on
arm64/riscv platform. But my new series takes a different way. And this
big lock is not a problem any longer.

Thanks for your time.

Regards,

	Pingfan