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:	Wed, 30 Sep 2015 13:50:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hidehiro Kawai <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,
	x86@...nel.org, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <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.

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