linux-kernel - Re: [PATCH] arm64:kexec: have own crash_smp_send

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Message-Id: <BB038411-7999-40CA-9454-7936368C0A80@gmail.com>
Date:   Fri, 4 Aug 2017 22:33:50 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64:kexec: have own crash_smp_send_stop() for crash dump for nonpanic cores


> On 4 Aug 2017, at 8:43 PM, AKASHI Takahiro <takahiro.akashi@...aro.org> wrote:
> 
>> On Fri, Aug 04, 2017 at 11:38:16AM +0100, James Morse wrote:
>> Hi Hoeun,
>> 
>>> On 04/08/17 08:02, Hoeun Ryu wrote:
>>> Commit 0ee5941 : (x86/panic: replace smp_send_stop() with kdump friendly
>>> version in panic path) introduced crash_smp_send_stop() which is a weak
>>> function and can be overriden by architecture codes to fix the side effect
>>> caused by commit f06e515 : (kernel/panic.c: add "crash_kexec_post_
>>> notifiers" option).
>> 
>> If I've understood correctly: if we boot with this option core code doesn't use
>> our machine_crash_shutdown(), and instead calls crash_smp_send_stop(), which we
> 
> No. Machine_crash_shutdown() is always called, but at that time,
> all the cpus other than the crashing cpu have already died in this case.
> 

You're right.

>> don't have, so it uses the default smp_send_stop(), which doesn't save the regs.
>> 
>> Thanks for catching this!
>> 
>> 
>> Could we rename smp_send_crash_stop() crash_smp_send_stop() and add the
>> called-twice logic there? They are similar enough that I'm getting them muddled
>> already!
>> 
> 
> Nice.

I'll reflect it in v2.
Thank you for the review.

> 
> -Takahiro AKASHI
> 
>> 
>> Thanks,
>> 
>> James
>> 
>> 
>>> ARM64 architecture uses the weak version function and the problem is that
>>> the weak function simply calls smp_send_stop() which makes other CPUs
>>> offline and takes away the chance to save crash information for nonpanic
>>> CPUs in machine_crash_shutdown() when crash_kexec_post_notifiers kernel
>>> option is enabled.
>>> 
>>> Calling smp_send_crash_stop() in the function is useless because all
>>> nonpanic CPUs are already offline by smp_send_stop() in this case and
>>> smp_send_crash_stop() only works against online CPUs.
>>> 
>>> The result is that /proc/vmcore is not available with the error messages;
>>> "Warning: Zero PT_NOTE entries found", "Kdump: vmcore not initialized".
>>> 
>>> crash_smp_send_stop() is implemented for ARM64 architecture to fix this
>>> problem and the function (strong symbol version) saves crash information
>>> for nonpanic CPUs using smp_send_crash_stop() and machine_crash_shutdown()
>>> tries to save crash information for nonpanic CPUs only when
>>> crash_kexec_post_notifiers kernel option is disabled.
>> 
>>