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

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Date:	Mon, 23 Mar 2015 11:36:59 -0400
From:	Vivek Goyal <vgoyal@...hat.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,
	masami.hiramatsu.pt@...achi.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: [PATCH v2] kernel/panic/kexec: fix "crash_kexec_post_notifiers"
 option issue in oops path

On Mon, Mar 23, 2015 at 08:19:43AM +0100, 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.
> 
> 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
> 

I quickly tested this patch to make sure I can still transition into
second kernel when crash_kexec_post_notifiers is specified on command
line. I have not tried running any notifiers. So.

Tested-by: Vivek Goyal <vgoyal@...hat.com>
Acked-by: Vivek Goyal <vgoyal@...hat.com>

This should be a general fix and not a replacement for the patch
in question in this mail thread. 

Thanks
Vivek

> 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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