linux-kernel - Re: [PATCH v2 1/1] powerpc/crash: Use NMI context for printk when starting to crash

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Date:   Wed, 13 May 2020 02:16:58 -0300
From:   Leonardo Bras <leobras.c@...il.com>
To:     Nicholas Piggin <npiggin@...il.com>,
        Alexios Zavras <alexios.zavras@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Enrico Weigelt <info@...ux.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 1/1] powerpc/crash: Use NMI context for printk when
 starting to crash

Hello Nick, thanks for your feedback.
Comments inline:

On Wed, 2020-05-13 at 14:36 +1000, Nicholas Piggin wrote:
> Excerpts from Leonardo Bras's message of May 13, 2020 7:45 am:
> > Currently, if printk lock (logbuf_lock) is held by other thread during
> > crash, there is a chance of deadlocking the crash on next printk, and
> > blocking a possibly desired kdump.
> > 
> > At the start of default_machine_crash_shutdown, make printk enter
> > NMI context, as it will use per-cpu buffers to store the message,
> > and avoid locking logbuf_lock.
> 
> printk_nmi_enter is used in one other place outside nmi_enter.
> 
> Is there a different/better way to handle this? What do other 
> architectures do?

To be honest, I was unaware of nmi_enter() and I have yet to study what
other architectures do here.

> Other subsystems get put into an nmi-mode when we call nmi_enter
> (lockdep, ftrace, rcu etc). It seems like those would be useful for 
> similar reasons, so at least explaining why that is not used in a 
> comment would be good.

My reasoning for using printk_nmi_enter() here was only to keep it from
using printk regular buffer (and locking logbuf_lock) at this point of
the crash.

I have yet to see how nmi_enter() extra functions would happen to
interfere with the crash at this point. 

(In a quick look at x86, (native_machine_crash_shutdown) I could not
see it using any printk, so it may not be necessary).

> Aside from that, I welcome any effort to make our crashes more reliable
> so thanks for working on this stuff.
> 
> Thanks,
> Nick

Thank you, it means a lot.

Leonardo Bras