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

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Message-ID: <04EAB7311EE43145B2D3536183D1A84454A3CDD3@GSjpTKYDCembx31.service.hitachi.net>
Date:	Thu, 3 Dec 2015 02:23:32 +0000
From:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>, Jonathan Corbet <corbet@....net>,
	"Peter Zijlstra" <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vivek Goyal <vgoyal@...hat.com>
CC:	Baoquan He <bhe@...hat.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>,
	"'Steven Rostedt'" <rostedt@...dmis.org>
Subject: RE: [V5 PATCH 2/4] panic/x86: Allow cpus to save registers even if
 they are looping in NMI context

> @@ -357,7 +358,15 @@ static void default_do_nmi(struct pt_regs *regs)
>  	}
> 
>  	/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
> -	raw_spin_lock(&nmi_reason_lock);
> +
> +	/*
> +	 * Another CPU may be processing panic routines with holding
> +	 * nmi_reason_lock.  Check IPI issuance from the panicking CPU
> +	 * and call the callback directly.
> +	 */
> +	while (!raw_spin_trylock(&nmi_reason_lock))
> +		poll_crash_ipi_and_callback(regs);
> +
>  	reason = x86_platform.get_nmi_reason();

I noticed this logic is unneeded until applying PATCH 4/4.
Currently, unknown NMI can be broadcast to all CPUs, but in that case,
panic()/nmi_panic() are called after releasing nmi_reason_lock.
So CPUs can't loop infinitely here.

PATCH 4/4 allows us to broadcast external NMIs to all CPUs, and it
causes infinite loop in raw_spin_lock(&nmi_reason_lock).  So the above
changes are needed.

I'll move these chagnes to a later patch in the next version.

Thanks,

--
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group