linux-kernel - Re: [V4 PATCH 2/2] mips/panic: Replace smp_send_stop() with kdump friendly version in panic path

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Date:	Mon, 15 Aug 2016 12:06:50 -0500
From:	Corey Minyard <cminyard@...sta.com>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>, Dave Young <dyoung@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Baoquan He <bhe@...hat.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"x86@...nel.org" <x86@...nel.org>,
	David Daney <david.daney@...ium.com>,
	Xunlei Pang <xpang@...hat.com>,
	Aaro Koskinen <aaro.koskinen@....fi>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	"linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	"Steven J. Hill" <steven.hill@...ium.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	Daniel Walker <dwalker@...o99.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [V4 PATCH 2/2] mips/panic: Replace smp_send_stop() with kdump
 friendly version in panic path

On 08/15/2016 06:35 AM, 河合英宏 / KAWAI，HIDEHIRO wrote:
> Hi Corey,
>
>> From: Corey Minyard [mailto:cminyard@...sta.com]
>> Sent: Friday, August 12, 2016 10:56 PM
>> I'll try to test this, but I have one comment inline...
> Thank you very much!
>
>> On 08/11/2016 10:17 PM, Dave Young wrote:
>>> On 08/10/16 at 05:09pm, Hidehiro Kawai wrote:
> [snip]
>>>> diff --git a/arch/mips/kernel/crash.c b/arch/mips/kernel/crash.c
>>>> index 610f0f3..1723b17 100644
>>>> --- a/arch/mips/kernel/crash.c
>>>> +++ b/arch/mips/kernel/crash.c
>>>> @@ -47,9 +47,14 @@ static void crash_shutdown_secondary(void *passed_regs)
>>>>
>>>>    static void crash_kexec_prepare_cpus(void)
>>>>    {
>>>> +	static int cpus_stopped;
>>>>    	unsigned int msecs;
>>>> +	unsigned int ncpus;
>>>>
>>>> -	unsigned int ncpus = num_online_cpus() - 1;/* Excluding the panic cpu */
>>>> +	if (cpus_stopped)
>>>> +		return;
>> Wouldn't you want an atomic operation and some special handling here to
>> ensure that only one CPU does this?  So if a CPU comes in here and
>> another CPU is already in the process stopping the CPUs it won't result in a
>> deadlock.
> Because this function can be called only one panicking CPU,
> there is no problem.
>
> There are two paths which crash_kexec_prepare_cpus is called.
>
> Path 1 (panic path):
> panic()
>    crash_smp_send_stop()
>      crash_kexec_prepare_cpus()
>
> Path 2 (oops path):
> crash_kexec()
>    __crash_kexec()
>      machine_crash_shutdown()
>        default_machine_crash_shutdown() // for MIPS
>          crash_kexec_prepare_cpus()
>
> Here, panic() and crash_kexec() run exclusively via
> panic_cpu atomic variable.  So we can use cpus_stopped as
> normal variable.

Ok, if the code can only be entered once, what's the purpose of 
cpus_stopped?
I guess that's what confused me.  You are right, the panic_cpu atomic should
keep this on a single CPU.

Also, panic() will call panic_smp_self_stop() if it finds another CPU 
has already
called panic, which will just spin with interrupts off by default. I 
didn't see a
definition for it in MIPS, wouldn't it need to be overridden to avoid a 
deadlock?

-corey

>
> Best regards,
>
> Hidehiro Kawai
>