linux-kernel - Re: [PATCH] sysrq, kdump: Make sysrq-c consistent

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Message-ID: <20090723104621.GA12838@hmsreliant.think-freely.org>
Date:	Thu, 23 Jul 2009 06:46:21 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, Lai Jiangshan <laijs@...fujitsu.com>,
	Ken'ichi Ohmichi <oomichi@....nes.nec.co.jp>,
	Vivek Goyal <vgoyal@...hat.com>,
	Brayan Arraes <brayan@...k.com.br>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sysrq, kdump: Make sysrq-c consistent

On Thu, Jul 23, 2009 at 02:12:10PM +0900, Hidetoshi Seto wrote:
> [ based on 2.6.31-rc4 ]
> 
> Recent commit d6580a9f15238b87e618310c862231ae3f352d2d changed the
> behavior of sysrq-c to unconditional dereference of NULL pointer.
> So in cases with CONFIG_KEXEC, where crash_kexec() was directly called
> from sysrq-c before, now it can be said that a step of "real oops" was
> inserted before starting kdump.
> 
> However, in contrast to oops via SysRq-c from keyboard which results
> in panic due to in_interrupt(), oops via "echo c > /proc/sysrq-trigger"
> will not become panic unless panic_on_oops=1.  It means that even if
> dump is properly configured to be taken on panic, the sysrq-c from proc
> interface might not start crashdump while the sysrq-c from keyboard can
> start crashdump.  This confuses traditional users of kdump, i.e. people
> who expect sysrq-c to do common behavior in both of the keyboard and
> proc interface.
> 
> This patch brings the keyboard and proc interface behavior of sysrq-c
> in line, by forcing panic_on_oops=1 before oops in sysrq-c handler.
> 
> And some updates in documentation are included, to clarify that there
> is no longer dependency with CONFIG_KEXEC, and that now the system can
> just crash by sysrq-c if no dump mechanism is configured.
> 
Acked-by: Neil Horman <nhorman@...driver.com>

Thanks!
Neil

> Signed-off-by: Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
> Cc: Lai Jiangshan <laijs@...fujitsu.com>
> Cc: Ken'ichi Ohmichi <oomichi@....nes.nec.co.jp>
> Cc: Neil Horman <nhorman@...driver.com>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: Brayan Arraes <brayan@...k.com.br>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  Documentation/sysrq.txt |    7 ++++---
>  drivers/char/sysrq.c    |    8 +++++---
>  2 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/sysrq.txt b/Documentation/sysrq.txt
> index cf42b82..d56a017 100644
> --- a/Documentation/sysrq.txt
> +++ b/Documentation/sysrq.txt
> @@ -66,7 +66,8 @@ On all -  write a character to /proc/sysrq-trigger.  e.g.:
>  'b'     - Will immediately reboot the system without syncing or unmounting
>            your disks.
>  
> -'c'	- Will perform a kexec reboot in order to take a crashdump.
> +'c'	- Will perform a system crash by a NULL pointer dereference.
> +          A crashdump will be taken if configured.
>  
>  'd'	- Shows all locks that are held.
>  
> @@ -141,8 +142,8 @@ useful when you want to exit a program that will not let you switch consoles.
>  re'B'oot is good when you're unable to shut down. But you should also 'S'ync
>  and 'U'mount first.
>  
> -'C'rashdump can be used to manually trigger a crashdump when the system is hung.
> -The kernel needs to have been built with CONFIG_KEXEC enabled.
> +'C'rash can be used to manually trigger a crashdump when the system is hung.
> +Note that this just triggers a crash if there is no dump mechanism available.
>  
>  'S'ync is great when your system is locked up, it allows you to sync your
>  disks and will certainly lessen the chance of data loss and fscking. Note
> diff --git a/drivers/char/sysrq.c b/drivers/char/sysrq.c
> index 0db3585..5d7a02f 100644
> --- a/drivers/char/sysrq.c
> +++ b/drivers/char/sysrq.c
> @@ -35,7 +35,6 @@
>  #include <linux/spinlock.h>
>  #include <linux/vt_kern.h>
>  #include <linux/workqueue.h>
> -#include <linux/kexec.h>
>  #include <linux/hrtimer.h>
>  #include <linux/oom.h>
>  
> @@ -124,9 +123,12 @@ static struct sysrq_key_op sysrq_unraw_op = {
>  static void sysrq_handle_crash(int key, struct tty_struct *tty)
>  {
>  	char *killer = NULL;
> +
> +	panic_on_oops = 1;	/* force panic */
> +	wmb();
>  	*killer = 1;
>  }
> -static struct sysrq_key_op sysrq_crashdump_op = {
> +static struct sysrq_key_op sysrq_crash_op = {
>  	.handler	= sysrq_handle_crash,
>  	.help_msg	= "Crash",
>  	.action_msg	= "Trigger a crash",
> @@ -401,7 +403,7 @@ static struct sysrq_key_op *sysrq_key_table[36] = {
>  	 */
>  	NULL,				/* a */
>  	&sysrq_reboot_op,		/* b */
> -	&sysrq_crashdump_op,		/* c & ibm_emac driver debug */
> +	&sysrq_crash_op,		/* c & ibm_emac driver debug */
>  	&sysrq_showlocks_op,		/* d */
>  	&sysrq_term_op,			/* e */
>  	&sysrq_moom_op,			/* f */
> -- 
> 1.6.0
> 
> 
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