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

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Message-ID: <20150324170417.GA5030@redhat.com>
Date:	Tue, 24 Mar 2015 13:04:17 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Baoquan He <bhe@...hat.com>,
	"Hatayama, Daisuke/畑山 大輔" 
	<d.hatayama@...fujitsu.com>, ebiederm@...ssion.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

On Tue, Mar 24, 2015 at 05:18:14PM +0100, Ingo Molnar wrote:
> 
> * Vivek Goyal <vgoyal@...hat.com> wrote:
> 
> > > Yet the actual bug is in that commit, 'crash_kexec_post_notifiers' 
> > > was clearly not a no-op in the default case, against expectations.
> > 
> > Hi Ingo,
> > 
> > I did a quick test and in default case crash_kexec() runs before 
> > panic notifiers. So it does look like crash_kexec_post_notifiers is 
> > a no-op in default case.
> > 
> > What am I missing.
> 
> Well, look at f06e5153f4ae:
> 
> diff --git a/kernel/panic.c b/kernel/panic.c
> index d02fa9fef46a..62e16cef9cc2 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -32,6 +32,7 @@ static unsigned long tainted_mask;
>  static int pause_on_oops;
>  static int pause_on_oops_flag;
>  static DEFINE_SPINLOCK(pause_on_oops_lock);
> +static bool crash_kexec_post_notifiers;
>  
>  int panic_timeout = CONFIG_PANIC_TIMEOUT;
>  EXPORT_SYMBOL_GPL(panic_timeout);
> @@ -112,9 +113,11 @@ void panic(const char *fmt, ...)
>  	/*
>  	 * If we have crashed and we have a crash kernel loaded let it handle
>  	 * everything else.
> -	 * Do we want to call this before we try to display a message?
> +	 * If we want to run this after calling panic_notifiers, pass
> +	 * the "crash_kexec_post_notifiers" option to the kernel.
>  	 */
> -	crash_kexec(NULL);
> +	if (!crash_kexec_post_notifiers)
> +		crash_kexec(NULL);
>  
>  	/*
>  	 * Note smp_send_stop is the usual smp shutdown function, which
> @@ -131,6 +134,15 @@ void panic(const char *fmt, ...)
>  
>  	kmsg_dump(KMSG_DUMP_PANIC);
>  
> +	/*
> +	 * If you doubt kdump always works fine in any situation,
> +	 * "crash_kexec_post_notifiers" offers you a chance to run
> +	 * panic_notifiers and dumping kmsg before kdump.
> +	 * Note: since some panic_notifiers can make crashed kernel
> +	 * more unstable, it can increase risks of the kdump failure too.
> +	 */
> +	crash_kexec(NULL);
> +
>  	bust_spinlocks(0);
>  
>  	if (!panic_blink)
> 
> 
> Without knowing what crash_kexec() does, the patch looks buggy: it 
> should preserve the old behavior by default, yet it will now execute a 
> second crash_kexec() after the kmsg_dump() line.
> 
> So the invariant change would have been to do:
> 
> -	crash_kexec(NULL);
> +	if (!crash_kexec_post_notifiers)
> +		crash_kexec(NULL);
> 
> ...
> 
> +	if (crash_kexec_post_notifiers)
> +		crash_kexec(NULL);
> 
> Which in the !crash_kexec_post_notifiers flag case reduces to:
> 
> 	crash_kexec();
> 
> 	...
> 
> 	/* NOP */
> 
> I.e. to exactly what the kernel was doing without the patch 
> originally.
> 
> Which is what my patch does. Nothing more, nothing less.

Ok, I got it what you mean.

crash_kexec() does not return if a kdump kernel is loaded. If kdump
kernel is not loaded, then crash_kexec() returns without doing anything.

I think that explains why not making second call to crash_kexec() under
if, did not create problems. In first case it will never be called and
in second case, it will do nothing and simply return back.

But anyway, we need your patch as that's right thing to do.

Thanks
Vivek
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