linux-kernel - Re: kdump: crash_kexec()-smp_send

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Message-ID: <m17h3tcm52.fsf@fess.ebiederm.org>
Date:	Tue, 25 Oct 2011 05:04:57 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	holzheu@...ux.vnet.ibm.com
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>, akpm@...ux-foundation.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: kdump: crash_kexec()-smp_send_stop() race in panic

Michael Holzheu <holzheu@...ux.vnet.ibm.com> writes:

> Hello Eric,
>
> On Mon, 2011-10-24 at 10:07 -0700, Eric W. Biederman wrote:
>
> [snip]
>
>> So my second thought is to introduce another atomic variable
>> panic_in_progress, visible only in panic.  The cpu that sets
>> increments panic_in_progress can call smp_send_stop.  The rest of
>> the cpus can just go into a busy wait.  That should stop nasty
>> fights about who is going to come out of smp_send_stop first.
>
> So this is a spinlock, no? What about the following patch:
Do we want both panic printks?

We really only need the mutual exclusion starting just before
smp_send_stop so that is where I would be inclined to put it.

But yeah something like the below should work.

Eric


> ---
>  kernel/panic.c |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -59,6 +59,7 @@ EXPORT_SYMBOL(panic_blink);
>   */
>  NORET_TYPE void panic(const char * fmt, ...)
>  {
> +	static DEFINE_SPINLOCK(panic_lock);
>  	static char buf[1024];
>  	va_list args;
>  	long i, i_next = 0;
> @@ -68,8 +69,12 @@ NORET_TYPE void panic(const char * fmt,
>  	 * It's possible to come here directly from a panic-assertion and
>  	 * not have preempt disabled. Some functions called from here want
>  	 * preempt to be disabled. No point enabling it later though...
> +	 *
> +	 * Only one CPU is allowed to execute the panic code. For multiple
> +	 * parallel invocations of panic all other CPUs will wait on the
> +	 * panic_lock. They are stopped afterwards by smp_send_stop().
>  	 */
> -	preempt_disable();
> +	spin_lock(&panic_lock);
>  
>  	console_verbose();
>  	bust_spinlocks(1);
--
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