Date:	Tue, 12 Jul 2016 14:57:24 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>,
	"'xlpang@...hat.com'" <xlpang@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <dyoung@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Baoquan He <bhe@...hat.com>
Cc:	Michal Hocko <mhocko@...e.com>, Toshi Kani <toshi.kani@....com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Minfei Huang <mnfhuang@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Daniel Walker <dwalker@...o99.com>,
	Ingo Molnar <mingo@...nel.org>,
	Takao Indoh <indou.takao@...fujitsu.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	Borislav Petkov <bp@...e.de>, Vivek Goyal <vgoyal@...hat.com>,
	Petr Mladek <pmladek@...e.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Tejun Heo <tj@...nel.org>,
	Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [V3 PATCH 1/2] x86/panic: Replace smp_send_stop() with kdump
 friendly version

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@...hat.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@...o99.com>
>>> Fixes: f06e5153f4ae (kernel/panic.c: add "crash_kexec_post_notifiers" option)
>>> Signed-off-by: Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
>>> Cc: Dave Young <dyoung@...hat.com>
>>> Cc: Baoquan He <bhe@...hat.com>
>>> Cc: Vivek Goyal <vgoyal@...hat.com>
>>> Cc: Eric Biederman <ebiederm@...ssion.com>
>>> Cc: Masami Hiramatsu <mhiramat@...nel.org>
>>> Cc: Thomas Gleixner <tglx@...utronix.de>
>>> Cc: Ingo Molnar <mingo@...hat.com>
>>> Cc: "H. Peter Anvin" <hpa@...or.com>
>>> Cc: Borislav Petkov <bp@...e.de>
>>> Cc: Toshi Kani <toshi.kani@....com>
>>> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
>>> Cc: Takao Indoh <indou.takao@...fujitsu.com>
>>> Cc: "Lee, Chun-Yi" <joeyli.kernel@...il.com>
>>> Cc: Minfei Huang <mnfhuang@...il.com>
>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>> Cc: Michal Hocko <mhocko@...e.com>
>>> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
>>> Cc: Petr Mladek <pmladek@...e.com>
>>> Cc: Tejun Heo <tj@...nel.org>
>>> Cc: Josh Poimboeuf <jpoimboe@...hat.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.

Regards,
Xunlei

>>> +
>>> +	if (cpus_stopped)
>>> +		return;
>>>
>>> +	nmi_shootdown_cpus(kdump_nmi_callback);
>>>  	disable_local_APIC();
>>> +	cpus_stopped = 1;
>>>  }
>>>
>>>  #else
>>> -static void kdump_nmi_shootdown_cpus(void)
>>> +void panic_smp_send_stop(void)
>>>  {
>>>  	/* There are no cpus to shootdown */
>>>  }
>>> @@ -160,7 +166,7 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
>>>  	/* The kernel is broken so disable interrupts */
>>>  	local_irq_disable();
>>>
>>> -	kdump_nmi_shootdown_cpus();
>>> +	panic_smp_send_stop();
>>>
>>>  	/*
>>>  	 * VMCLEAR VMCSs loaded on this cpu if needed.
>>> diff --git a/kernel/panic.c b/kernel/panic.c
>>> index 8aa7449..da8062d2 100644
>>> --- a/kernel/panic.c
>>> +++ b/kernel/panic.c
>>> @@ -71,6 +71,32 @@ void __weak nmi_panic_self_stop(struct pt_regs *regs)
>>>  	panic_smp_self_stop();
>>>  }
>>>
>>> +/*
>>> + * Stop other CPUs in panic.  Architecture dependent code may override this
>>> + * with more suitable version.  For example, if the architecture supports
>>> + * crash dump, it should save registers of each stopped CPU and disable
>>> + * per-CPU features such as virtualization extensions.
>>> + */
>>> +void __weak panic_smp_send_stop(void)
>>> +{
>>> +	static int cpus_stopped;
>>> +
>>> +	/*
>>> +	 * This function can be called twice in panic path, but obviously
>>> +	 * we execute this only once.
>>> +	 */
>>> +	if (cpus_stopped)
>>> +		return;
>>> +
>>> +	/*
>>> +	 * Note smp_send_stop is the usual smp shutdown function, which
>>> +	 * unfortunately means it may not be hardened to work in a panic
>>> +	 * situation.
>>> +	 */
>>> +	smp_send_stop();
>>> +	cpus_stopped = 1;
>>> +}
>>> +
>>>  atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID);
>>>
>>>  /*
>>> @@ -125,7 +151,7 @@ void panic(const char *fmt, ...)
>>>  	 * Only one CPU is allowed to execute the panic code from here. For
>>>  	 * multiple parallel invocations of panic, all other CPUs either
>>>  	 * stop themself or will wait until they are stopped by the 1st CPU
>>> -	 * with smp_send_stop().
>>> +	 * with panic_smp_send_stop().
>>>  	 *
>>>  	 * `old_cpu == PANIC_CPU_INVALID' means this is the 1st CPU which
>>>  	 * comes here, so go ahead.
>>> @@ -165,12 +191,7 @@ void panic(const char *fmt, ...)
>>>  		__crash_kexec(NULL);
>>>  	}
>>>
>>> -	/*
>>> -	 * Note smp_send_stop is the usual smp shutdown function, which
>>> -	 * unfortunately means it may not be hardened to work in a panic
>>> -	 * situation.
>>> -	 */
>>> -	smp_send_stop();
>>> +	panic_smp_send_stop();
>> If crash_kexec_post_notifiers and kdump both are not used, you actually changed the original behaviour,
>> it would be better to make it conditionally like:
>>     if (crash_kexec_post_notifiers)
>>         panic_smp_send_stop(); // renaming it to crash_smp_send_stop() is better?
>>     else
>>         smp_send_stop();
> Dave also pointed about this, and I'll fix it.
>
> Best regards,
>
> Hidehiro Kawai
> Hitachi, Ltd. Research & Development Group
>
>>>  	/*
>>>  	 * Run any panic handlers, including those that might need to
>>> @@ -198,10 +219,10 @@ void panic(const char *fmt, ...)
>>>
>>>  	/*
>>>  	 * We may have ended up stopping the CPU holding the lock (in
>>> -	 * smp_send_stop()) while still having some valuable data in the console
>>> -	 * buffer.  Try to acquire the lock then release it regardless of the
>>> -	 * result.  The release will also print the buffers out.  Locks debug
>>> -	 * should be disabled to avoid reporting bad unlock balance when
>>> +	 * panic_smp_send_stop()) while still having some valuable data in the
>>> +	 * console buffer.  Try to acquire the lock then release it regardless
>>> +	 * of the result.  The release will also print the buffers out.  Locks
>>> +	 * debug should be disabled to avoid reporting bad unlock balance when
>>>  	 * panic() is not being callled from OOPS.
>>>  	 */
>>>  	debug_locks_off();
>>>
>>>
>>>
>>> _______________________________________________
>>> kexec mailing list
>>> kexec@...ts.infradead.org
>>> http://lists.infradead.org/mailman/listinfo/kexec

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