linux-kernel - Re: [V3 PATCH 3/4] kexec: Fix race between panic() and crash

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Date:	Tue, 25 Aug 2015 16:52:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
Cc:	Jonathan Corbet <corbet@....net>, 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>,
	"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>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
Subject: Re: [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()
 called directly

On Sat, Aug 22, 2015 at 02:35:24AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > From: Peter Zijlstra [mailto:peterz@...radead.org]
> > 
> > On Thu, Aug 06, 2015 at 02:45:43PM +0900, Hidehiro Kawai wrote:
> > >  void crash_kexec(struct pt_regs *regs)
> > >  {
> > > +	int old_cpu, this_cpu;
> > > +
> > > +	/*
> > > +	 * `old_cpu == -1' means we are the first comer and crash_kexec()
> > > +	 * was called without entering panic().
> > > +	 * `old_cpu == this_cpu' means crash_kexec() was called from panic().
> > > +	 */
> > > +	this_cpu = raw_smp_processor_id();
> > > +	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > > +	if (old_cpu != -1 && old_cpu != this_cpu)
> > > +		return;
> > 
> > This allows recursive calling of crash_kexec(), the Changelog did not
> > mention that. Is this really required?
> 
> What part are you arguing?  Recursive call of crash_kexec() doesn't
> happen.  In the first place, one of the purpose of this patch is
> to prevent a recursive call of crash_kexec() in the following case
> as I stated in the description:
> 
> CPU 0:
>   oops_end()
>     crash_kexec()
>       mutex_trylock() // acquired
>         <NMI>
>         io_check_error()
>           panic()
>             crash_kexec()
>               mutex_trylock() // failed to acquire
>             infinite loop
> 

Yes, but what to we want to do there? It seems to me that is wrong, we
do not want to let a recursive crash_kexec() proceed.

Whereas the condition you created explicitly allows this recursion by
virtue of the 'old_cpu != this_cpu' check.

You changelog does not explain why you want a recursive crash_kexec().

> Also, the logic doesn't change form V1 (although the implementation
> changed), so I didn't add changelogs any more.

I cannot remember V1, nor can any prior patch be relevant.

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