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

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Date:	Sat, 22 Aug 2015 02:35:24 +0000
From:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
To:	"'Peter Zijlstra'" <peterz@...radead.org>
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

> 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


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

Regards,

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group