linux-kernel - Re: [PATCH v5 2/2] x86 mce: use new printk recursion disabling interface

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Message-ID: <20120605151542.GA10669@x1.osrc.amd.com>
Date:	Tue, 5 Jun 2012 17:15:42 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	ShuoX Liu <shuox.liu@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	"Luck, Tony" <tony.luck@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v5 2/2] x86 mce: use new printk recursion disabling
 interface

On Tue, Jun 05, 2012 at 05:55:22PM +0800, ShuoX Liu wrote:
> From: ShuoX Liu <shuox.liu@...el.com>
> 
> On x86 machines, some times MCE happens just when kernel calls printk
> to output some log info to serial console, while usually MCE module in
> kernel is used to print out some hardware error information, such like
> bad cache or bad memory bank. That causes printk recursion and printk
> would omit MCE printk output.
> 
> We hit it when running MTBF testing on Android ATOM mobiles.
> 
> Here in mce_panic, we choose to disable printk recursion to make sure
> MCE logs printed out.
> 
> Signed-off-by: Yanmin Zhang <yanmin_zhang@...ux.intel.com>
> Signed-off-by: ShuoX Liu <shuox.liu@...el.com>
> ---
>  arch/x86/kernel/cpu/mcheck/mce.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index 2afcbd2..906e838 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -306,6 +306,7 @@ static void mce_panic(char *msg, struct mce *final, char *exp)
>  {
>  	int i, apei_err = 0;
>  
> +	printk_recursion_check_disable();
>  	if (!fake_panic) {
>  		/*
>  		 * Make sure only one CPU runs in machine check panic
> @@ -360,6 +361,7 @@ static void mce_panic(char *msg, struct mce *final, char *exp)
>  		panic(msg);
>  	} else
>  		pr_emerg(HW_ERR "Fake kernel panic: %s\n", msg);
> +	printk_recursion_check_enable();

Ok, let me ask this again: why not disable the printk recursion check in
the function that actually _prints_ the MCE, i.e. print_mce() instead of
here in mce_panic() which does a whole bunch of other stuff and it can
also return without printing any MCE to dmesg?

Are you interested in seeing the printk's from mce_panic? Why?

Thanks.

-- 
Regards/Gruss,
Boris.
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