Re: [V5 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI

From: Michal Hocko <mhocko@kernel.org>
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,
	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>,
	Borislav Petkov <bp@alien8.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>, Vivek Goyal <vgoyal@redhat.com>
Subject: Re: [V5 PATCH 1/4] panic/x86: Fix re-entrance problem due to panic on NMI
Date: Tue, 24 Nov 2015 21:27:13 +0100	[thread overview]
Message-ID: <20151124202712.GA23063@dhcp22.suse.cz> (raw)
In-Reply-To: <20151124150510.GA6100@home.goodmis.org>

On Tue 24-11-15 10:05:10, Steven Rostedt wrote:
> On Fri, Nov 20, 2015 at 06:36:44PM +0900, Hidehiro Kawai wrote:
> > diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> > index 350dfb0..480a4fd 100644
> > --- a/include/linux/kernel.h
> > +++ b/include/linux/kernel.h
> > @@ -445,6 +445,19 @@ extern int sysctl_panic_on_stackoverflow;
> >  
> >  extern bool crash_kexec_post_notifiers;
> >  
> > +extern atomic_t panic_cpu;
> > +
> > +/*
> > + * A variant of panic() called from NMI context.
> > + * If we've already panicked on this cpu, return from here.
> > + */
> > +#define nmi_panic(fmt, ...)						\
> > +	do {								\
> > +		int this_cpu = raw_smp_processor_id();			\
> > +		if (atomic_cmpxchg(&panic_cpu, -1, this_cpu) != this_cpu) \
> > +			panic(fmt, ##__VA_ARGS__);			\
> 
> Hmm,
> 
> What happens if:
> 
> 	CPU 0:				CPU 1:
> 	------				------
> 	nmi_panic();
> 
> 					nmi_panic();
> 					<external nmi>
> 					nmi_panic();

I thought that nmi_panic is called only from the nmi context. If so how
can we get a nested NMI like that?
-- 
Michal Hocko
SUSE Labs

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