linux-kernel - Re: Re: [PATCH v2] kernel/panic/kexec: fix "crash_kexec_post

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Date:	Tue, 24 Mar 2015 12:30:10 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Baoquan He <bhe@...hat.com>,
	"Hatayama, Daisuke/畑山 大輔" <d.hatayama@...fujitsu.com>,
	ebiederm@...ssion.com, Vivek Goyal <vgoyal@...hat.com>,
	hidehiro.kawai.ez@...achi.com, linux-kernel@...r.kernel.org,
	kexec@...ts.infradead.org, akpm@...ux-foundation.org,
	mingo@...hat.com, bp@...e.de
Subject: Re: Re: [PATCH v2] kernel/panic/kexec: fix "crash_kexec_post_notifiers"
 option issue in oops path

(2015/03/23 16:19), Ingo Molnar wrote:
> 
> * Baoquan He <bhe@...hat.com> wrote:
> 
>> CC more people ...
>>
>> On 03/07/15 at 01:31am, "Hatayama, Daisuke/畑山 大輔" wrote:
>>> The commit f06e5153f4ae2e2f3b0300f0e260e40cb7fefd45 introduced
>>> "crash_kexec_post_notifiers" kernel boot option, which toggles
>>> wheather panic() calls crash_kexec() before panic_notifiers and dump
>>> kmsg or after.
>>>
>>> The problem is that the commit overlooks panic_on_oops kernel boot
>>> option. If it is enabled, crash_kexec() is called directly without
>>> going through panic() in oops path.
>>>
>>> To fix this issue, this patch adds a check to
>>> "crash_kexec_post_notifiers" in the condition of kexec_should_crash().
>>>
>>> Also, put a comment in kexec_should_crash() to explain not obvious
>>> things on this patch.
>>>
>>> Signed-off-by: HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
>>> Acked-by: Baoquan He <bhe@...hat.com>
>>> Tested-by: Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>
>>> Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
>>> ---
>>>  include/linux/kernel.h |  3 +++
>>>  kernel/kexec.c         | 11 +++++++++++
>>>  kernel/panic.c         |  2 +-
>>>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> This is hack upon hack, but why was this crap merged in the first 
> place?
> 
> I see two problems just by cursory review:
> 
> 1)
> 
> Firstly, the real bug in:
> 
>   f06e5153f4ae ("kernel/panic.c: add "crash_kexec_post_notifiers" option for kdump after panic_notifers")
> 
> Was that crash_kexec() was called unconditionally after notifiers were 
> called, which should be fixed via the simple patch below (untested). 
> Looks much simpler than your fix.

No, Daisuke's patch is not for that case. Since the kdump has a special hook in
kernel oops, when both of panic_on_oops and crash_kernel are set, panic() is
never called. Please see oops_end@...h/x86/kernel/dumpstack.c

----
void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
{
        if (regs && kexec_should_crash(current))
                crash_kexec(regs);
----
Of course crash_kexec() never return except failing kexec unexpectedly.

Thus, kexec_should_crash should returns 0 if crash_kexec_post_notifiers is set.
(Semantically, it is a bit strange that panic_on_oops doesn't call panic(), but
that is another topic.)

However, your patch is also needed since the first crash_kexec() can fail in panic()
when crash_kexec_post_notifiers is not set. In that case, kernel tries to call
notifiers and call the 2nd crash_kexec() again. Actually the 2nd one is useless.

So, here is my reviewed-by.

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>

I'll be reply the latter part in other mail.

Thank you,

> 
> 2)
> 
> Secondly, and more importantly, the whole premise of commit 
> f06e5153f4ae is broken IMHO:
> 
>  "This can help rare situations where kdump fails because of unstable
>   crashed kernel or hardware failure (memory corruption on critical
>   data/code)"
> 
> wtf?
> 
> If the kernel crashed due to a kernel crash, then the kernel booting 
> up in whatever hardware state should be able to do a clean bootup. The 
> fix for those 'rare situations' should be to fix the real bug (for 
> example by making hardware driver init (or deinit) sequences more 
> robust), not to paper it over by ordering around crash-time sequences 
> ...
> 
> If it crashed due to some hardware failure, there's literally an 
> infinite amount of failure modes that may or may not be impacted by 
> kexec crash-time handling ordering. We don't want to put a zillion 
> such flags into the kernel proper just to allow the perturbation of 
> the kernel.
> 
> Thanks,
> 
> 	Ingo
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 8136ad76e5fd..774614f72cbd 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -142,7 +142,8 @@ void panic(const char *fmt, ...)
>  	 * Note: since some panic_notifiers can make crashed kernel
>  	 * more unstable, it can increase risks of the kdump failure too.
>  	 */
> -	crash_kexec(NULL);
> +	if (crash_kexec_post_notifiers)
> +		crash_kexec(NULL);
>  
>  	bust_spinlocks(0);
>  
> --
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> 


-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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