Re: [V3 PATCH 1/2] x86/panic: Replace smp_send_stop() with kdump friendly version

[Xunlei Pang <xpang@redhat.com>]

On 2016/07/12 at 15:12, 河合英宏 / KAWAI，HIDEHIRO wrote:
>> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Xunlei Pang
>> Sent: Tuesday, July 12, 2016 3:57 PM
>> On 2016/07/12 at 11:56, 河合英宏 / KAWAI，HIDEHIRO wrote:
>>> Hi Xunlei,
>>>
>>> Thanks for the review.
>>>
>>>> From: Xunlei Pang [mailto:xpang@redhat.com]
>>>> Sent: Tuesday, July 12, 2016 12:12 PM
>>>> On 2016/07/05 at 19:33, Hidehiro Kawai wrote:
>>>>> This patch fixes one of the problems reported by Daniel Walker
>>>>> (https://lkml.org/lkml/2015/6/24/44).
>>>>>
>>>>> If crash_kexec_post_notifiers boot option is specified, other CPUs
>>>>> are stopped by smp_send_stop() instead of machine_crash_shutdown()
>>>>> in crash_kexec() path.  This behavior change leads two problems.
>>>>>
>>>>>  Problem 1:
>>>>>  octeon_generic_shutdown() for MIPS OCTEON assumes that other CPUs are
>>>>>  still online and try to stop their watchdog timer.  If
>>>>>  smp_send_stop() is called before octeon_generic_shutdown(), stopping
>>>>>  watchdog timer will fail because other CPUs have been offlined by
>>>>>  smp_send_stop().
>>>>>
>>>>>    panic()
>>>>>      if crash_kexec_post_notifiers == 1
>>>>>        smp_send_stop()
>>>>>        atomic_notifier_call_chain()
>>>>>        kmsg_dump()
>>>>>      crash_kexec()
>>>>>        machine_crash_shutdown()
>>>>>          octeon_generic_shutdown() // shutdown watchdog for ONLINE CPUs
>>>>>
>>>>>  Problem 2:
>>>>>  Most of architectures stop other CPUs in machine_crash_shutdown()
>>>>>  path, and they also do something needed for kdump.  For example,
>>>>>  they save registers, disable virtualization extensions, and so on.
>>>>>  However, if smp_send_stop() stops other CPUs before
>>>>>  machine_crash_shutdown(), we miss those operations.
>>>>>
>>>>> How do we fix these problems?  In the first place, we should stop
>>>>> other CPUs as soon as possible when panic() was called, otherwise
>>>>> other CPUs may wipe out a clue to the cause of the failure.  So, we
>>>>> replace smp_send_stop() with more suitable one for kdump.
>>>>>
>>>>> This patch solves Problem 2 by replacing smp_send_stop() in panic()
>>>>> with panic_smp_send_stop().  This is a weak function which calls
>>>>> smp_send_stop(), and architecture dependent code may override this
>>>>> with appropriate one.  This patch only provides x86-specific version.
>>>>>
>>>>> Changes in V3:
>>>>> - Revise comments, description, and symbol names
>>>>>
>>>>> Changes in V2:
>>>>> - Replace smp_send_stop() call with crash_kexec version which
>>>>>   saves cpu states and cleans up VMX/SVM
>>>>> - Drop a fix for Problem 1 at this moment
>>>>>
>>>>> Reported-by: Daniel Walker <dwalker@fifo99.com>
>>>>> Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" option)
>>>>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com>
>>>>> Cc: Dave Young <dyoung@redhat.com>
>>>>> Cc: Baoquan He <bhe@redhat.com>
>>>>> Cc: Vivek Goyal <vgoyal@redhat.com>
>>>>> Cc: Eric Biederman <ebiederm@xmission.com>
>>>>> Cc: Masami Hiramatsu <mhiramat@kernel.org>
>>>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>>>> Cc: Ingo Molnar <mingo@redhat.com>
>>>>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>>>>> Cc: Borislav Petkov <bp@suse.de>
>>>>> Cc: Toshi Kani <toshi.kani@hpe.com>
>>>>> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
>>>>> Cc: Takao Indoh <indou.takao@jp.fujitsu.com>
>>>>> Cc: "Lee, Chun-Yi" <joeyli.kernel@gmail.com>
>>>>> Cc: Minfei Huang <mnfhuang@gmail.com>
>>>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>>>> Cc: Michal Hocko <mhocko@suse.com>
>>>>> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
>>>>> Cc: Petr Mladek <pmladek@suse.com>
>>>>> Cc: Tejun Heo <tj@kernel.org>
>>>>> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
>>>>> ---
>>>>>  arch/x86/kernel/crash.c |   14 ++++++++++----
>>>>>  kernel/panic.c          |   43 ++++++++++++++++++++++++++++++++-----------
>>>>>  2 files changed, 42 insertions(+), 15 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kernel/crash.c b/arch/x86/kernel/crash.c
>>>>> index 9ef978d..3305433 100644
>>>>> --- a/arch/x86/kernel/crash.c
>>>>> +++ b/arch/x86/kernel/crash.c
>>>>> @@ -133,15 +133,21 @@ static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
>>>>>  	disable_local_APIC();
>>>>>  }
>>>>>
>>>>> -static void kdump_nmi_shootdown_cpus(void)
>>>>> +/* Override the weak function in kernel/panic.c */
>>>>> +void panic_smp_send_stop(void)
>>>>>  {
>>>>> -	nmi_shootdown_cpus(kdump_nmi_callback);
>>>>> +	static int cpus_stopped;
>>>> Should be atomic_t type?
>>> panic_smp_send_stop() can be called by only one panicking CPU
>>> (but can be called twice). It is sufficient to be normal variable.
>> There are other call sites of __crash_kexec() for oops cases, which can
>> call panic_smp_send_stop() concurrently on a different cpu.
> In oops cases, crash_kexec() is called first, then __crash_kexec() is
> called.  crash_kexec() excludes concurrent execution of panic() and
> crash_kexec() via panic_cpu, so panic_smp_send_stop() shouldn't be
> called concurrently.

Right, that's why oops calls crash_kexec() not __crash_kexec().
I have no problem on this. Thanks!

Regards,
Xunlei

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