Re: [V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context

From: Borislav Petkov <bp@alien8.de>
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>
Subject: Re: [V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context
Date: Tue, 24 Nov 2015 21:16:41 +0100	[thread overview]
Message-ID: <20151124201641.GD21613@pd.tnic> (raw)
In-Reply-To: <20151124193700.GB6100@home.goodmis.org>

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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