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Date: Tue, 24 Nov 2015 21:16:41 +0100
From: Borislav Petkov <bp@alien8.de>
Subject: Re: [V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if
 they are looping in NMI context
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To: Steven Rostedt <rostedt@goodmis.org>
Cc: x86@kernel.org, Baoquan He <bhe@redhat.com>, Jonathan Corbet <corbet@lwn.net>, Peter Zijlstra <peterz@infradead.org>, linux-doc@vger.kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>, Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>, "Eric W. Biederman" <ebiederm@xmission.com>, "H. Peter Anvin" <hpa@zytor.com>, Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>, Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@kernel.org>, Vivek Goyal <vgoyal@redhat.com>

On Tue, Nov 24, 2015 at 02:37:00PM -0500, Steven Rostedt wrote:
> On Tue, Nov 24, 2015 at 11:48:53AM +0100, Borislav Petkov wrote:
> > 
> > > +	 */
> > > +	while (!raw_spin_trylock(&nmi_reason_lock))
> > > +		poll_crash_ipi_and_callback(regs);
> > 
> > Waaait a minute: so if we're getting NMIs broadcasted on every core but
> > we're *not* crash dumping, we will run into here too. This can't be
> > right. :-\
> 
> This only does something if crash_ipi_done is set, which means you are killing
> the box.

Yeah, Michal and I discussed that on IRC today. And yeah, it is really
tricky stuff. So I appreciate it a lot you looking at it too. Thanks!

> But perhaps a comment that states that here would be useful, or maybe
> just put in the check here. There's no need to make it depend on SMP, as
> raw_spin_trylock() will turn to just ({1}) for UP, and that code wont even be
> hit.

Right, this code needs much more thorough documentation to counter the
trickiness.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

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