linux-kernel - Re: [PATCH v4 2/2] x86 mce: use new printk recursion disabling interface

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Message-Id: <20120622164143.bb7dc290.akpm@linux-foundation.org>
Date:	Fri, 22 Jun 2012 16:41:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	shuox.liu@...el.com
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	andi@...stfloor.org, Tony Luck <tony.luck@...el.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v4 2/2] x86 mce: use new printk recursion disabling
 interface

On Mon, 04 Jun 2012 11:07:11 +0800
ShuoX Liu <shuox.liu@...el.com> wrote:

> From: ShuoX Liu <shuox.liu@...el.com>
> 
> 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>
> ---
> We hit it when running a MTBF testing on a Android atom mobile.
> ---
>  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();
>  }
>  
>  /* Support code for software error injection */

A couple of things here.

a) mce_panic() has a "return" statement deep inside.  So we return
   from mce_panic() with the recursion check disabled.  whoops.

b) adding a nice comment is nice.

--- a/arch/x86/kernel/cpu/mcheck/mce.c~x86-mce-use-new-printk-recursion-disabling-interface-fix
+++ a/arch/x86/kernel/cpu/mcheck/mce.c
@@ -303,11 +303,10 @@ static void wait_for_panic(void)
 	panic("Panicing machine check CPU died");
 }
 
-static void mce_panic(char *msg, struct mce *final, char *exp)
+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
@@ -362,6 +361,17 @@ static void mce_panic(char *msg, struct 
 		panic(msg);
 	} else
 		pr_emerg(HW_ERR "Fake kernel panic: %s\n", msg);
+}
+
+/*
+ * If an MCE happens to occur during the execution of a printk(), we want the
+ * MCE information to be displayed.  But printk()'s recursion checking prevents
+ * that.  So temporarily disable it.
+ */
+static void mce_panic(char *msg, struct mce *final, char *exp)
+{
+	printk_recursion_check_disable();
+	__mce_panic(msg, final, exp);
 	printk_recursion_check_enable();
 }
 
_

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