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Message-ID: <20220511111320.GB26047@pathway.suse.cz>
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>,
        Evan Green <evgreen@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>, bhe@...hat.com,
        kexec@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        bcm-kernel-feedback-list@...adcom.com,
        linuxppc-dev@...ts.ozlabs.org, linux-alpha@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-leds@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, Linux PM <linux-pm@...r.kernel.org>,
        linux-remoteproc@...r.kernel.org, linux-s390@...r.kernel.org,
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        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
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        luto@...nel.org, mhiramat@...nel.org, mingo@...hat.com,
        paulmck@...nel.org, peterz@...radead.org, senozhatsky@...omium.org,
        Alan Stern <stern@...land.harvard.edu>,
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        vkuznets@...hat.com, Will Deacon <will@...nel.org>,
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        David Gow <davidgow@...gle.com>,
        Julius Werner <jwerner@...omium.org>
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

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