From: =?utf-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= 
	<hidehiro.kawai.ez@hitachi.com>
To: =?utf-8?B?5rKz5ZCI6Iux5a6PIC8gS0FXQUnvvIxISURFSElSTw==?= 
	<hidehiro.kawai.ez@hitachi.com>,
        "'Peter Zijlstra'" <peterz@infradead.org>
CC: Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Vivek Goyal <vgoyal@redhat.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        "kexec@lists.infradead.org" <kexec@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        =?utf-8?B?5bmz5p2+6ZuF5bezIC8gSElSQU1BVFXvvIxNQVNBTUk=?= 
	<masami.hiramatsu.pt@hitachi.com>
Subject: RE: [V3 PATCH 3/4] kexec: Fix race between panic() and
 crash_kexec() called directly
Thread-Topic: [V3 PATCH 3/4] kexec: Fix race between panic() and
 crash_kexec() called directly
Thread-Index: AQHQ0AtjXekgeO133USkzCimPcfuqJ4VA1yAgAJdTKCABPPXAIABVqwggAhInsA=
Date: Mon, 31 Aug 2015 08:53:11 +0000
Message-ID: <04EAB7311EE43145B2D3536183D1A8445495793E@GSjpTKYDCembx31.service.hitachi.net>
References: <20150806054543.25766.29590.stgit@softrs>
 <20150806054543.25766.5914.stgit@softrs>
 <20150820230845.GF3161@worktop.event.rightround.com>
 <04EAB7311EE43145B2D3536183D1A8445493C868@GSjpTKYDCembx31.service.hitachi.net>
 <20150825145258.GS16853@twins.programming.kicks-ass.net>
 <04EAB7311EE43145B2D3536183D1A84454951595@GSjpTKYDCembx31.service.hitachi.net>
In-Reply-To: <04EAB7311EE43145B2D3536183D1A84454951595@GSjpTKYDCembx31.service.hitachi.net>
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Hello Peter,

> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of 河合英宏 / KAWAI，
> 
> Hi,
> 
> > From: Peter Zijlstra [mailto:peterz@infradead.org]
> >
> > On Sat, Aug 22, 2015 at 02:35:24AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > > > From: Peter Zijlstra [mailto:peterz@infradead.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.
> 
> I understand your question.  I don't intend to permit the recursive
> call of crash_kexec() as for 'old_cpu != this_cpu' check.  That is
> needed for the case of panic() --> crash_kexec().  Since panic_cpu has
> already been set to this_cpu in panic() (please see PATCH 1/4), no one
> can run crash_kexec() without 'old_cpu != this_cpu' check.
> 
> If you don't like this check, I would also be able to handle this case
> like below:
> 
> crash_kexec()
> {
> 	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> 	if (old_cpu != -1)
> 		return;
> 
> 	__crash_kexec();
> }
> 
> panic()
> {
> 	atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> 	__crash_kexec();
> ...
> 

Is that OK?

Regards,

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group


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