linux-kernel - RE: [V4 PATCH 2/4] panic/x86: Allow cpus to save registers even if they are looping in NMI context

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Date:	Thu, 1 Oct 2015 01:43:18 +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>,
	Ingo Molnar <mingo@...hat.com>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
Subject: RE: [V4 PATCH 2/4] panic/x86: Allow cpus to save registers even if
 they are looping in NMI context

> On Fri, Sep 25, 2015 at 08:28:07PM +0900, Hidehiro Kawai wrote:
> > --- a/arch/x86/kernel/reboot.c
> > +++ b/arch/x86/kernel/reboot.c
> > @@ -718,6 +718,7 @@ void machine_crash_shutdown(struct pt_regs *regs)
> >  static nmi_shootdown_cb shootdown_callback;
> >
> >  static atomic_t waiting_for_crash_ipi;
> > +static int crash_ipi_done;
> >
> >  static int crash_nmi_callback(unsigned int val, struct pt_regs *regs)
> >  {
> > @@ -779,6 +780,7 @@ void nmi_shootdown_cpus(nmi_shootdown_cb callback)
> >  	wmb();
> >
> >  	smp_send_nmi_allbutself();
> > +	crash_ipi_done = 1; /* Kick cpus looping in nmi context */
> 
> I would suggest using WRITE_ONCE() for that, because without the
> volatile the compiler need not actually emit the store until after the
> whole waiting thing _IF_ it can inline the whole thing.
> 
> Currently udelay() will end up being a function call and will therefore
> force the store to be emitted, but I'd rather not rely on that.

OK, I use WRITE_ONCE().
Thanks!

> >
> >  	msecs = 1000; /* Wait at most a second for the other cpus to stop */
> >  	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {


Hidehiro Kawai
Hitachi, Ltd. Research & Development Group