netdev - Re: [PATCH 04/30] firmware: google: Convert regular spinlock into trylock on panic path

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Date:   Wed, 11 May 2022 13:13:20 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
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Subject: Re: [PATCH 04/30] firmware: google: Convert regular spinlock into
 trylock on panic path

On Tue 2022-05-10 21:46:38, John Ogness wrote:
> On 2022-05-10, Steven Rostedt <rostedt@...dmis.org> wrote:
> >> As already mentioned in the other reply, panic() sometimes stops the
> >> other CPUs using NMI, for example, see kdump_nmi_shootdown_cpus().
> >> 
> >> Another situation is when the CPU using the lock ends in some
> >> infinite loop because something went wrong. The system is in
> >> an unpredictable state during panic().
> >> 
> >> I am not sure if this is possible with the code under gsmi_dev.lock
> >> but such things really happen during panic() in other subsystems.
> >> Using trylock in the panic() code path is a good practice.
> >
> > I believe that Peter Zijlstra had a special spin lock for NMIs or
> > early printk, where it would not block if the lock was held on the
> > same CPU. That is, if an NMI happened and paniced while this lock was
> > held on the same CPU, it would not deadlock. But it would block if the
> > lock was held on another CPU.
> 
> Yes. And starting with 5.19 it will be carrying the name that _you_ came
> up with (cpu_sync):
> 
> printk_cpu_sync_get_irqsave()
> printk_cpu_sync_put_irqrestore()

There is a risk that this lock might become a big kernel lock.

This special lock would need to be used even during normal
system operation. It does not make sense to suddenly start using
another lock during panic.

So I think that we should think twice before using it.
I would prefer using trylock of the original lock when
possible during panic.

It is possible that I miss something.

Best Regards,
Petr