linux-kernel - Re: [PATCH] x86: Avoid intermixing cpu dump_stack output on multi-processor systems

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Date:	Tue, 29 May 2012 13:53:53 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Russ Anderson <rja@....com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] x86: Avoid intermixing cpu dump_stack output on
 multi-processor systems

On Thu, May 24, 2012 at 09:42:29AM -0500, Russ Anderson wrote:
> When multiple cpus on a multi-processor system call dump_stack()
> at the same time, the backtrace lines get intermixed, making 
> the output worthless.  Add a lock so each cpu stack dump comes
> out as a coherent set.
> 
> For example, when a multi-processor system is NMIed, all of the
> cpus call dump_stack() at the same time, resulting in output for
> all of cpus getting intermixed, making it impossible to tell what
> any individual cpu was doing.  With this patch each cpu prints
> its stack lines as a coherent set, so one can see what each cpu
> was doing.

For this particular test case, it sounds like you are doing what
trigger_all_cpu_backtrace() is doing?  It doesn't solve the general
problem, but probably your particular usage?

Cheers,
Don

> 
> It has been tested on a 4069 cpu system.
> 
> Signed-off-by: Russ Anderson <rja@....com>
> 
> ---
>  arch/x86/kernel/dumpstack.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> Index: linux/arch/x86/kernel/dumpstack.c
> ===================================================================
> --- linux.orig/arch/x86/kernel/dumpstack.c	2012-05-03 14:31:13.602345805 -0500
> +++ linux/arch/x86/kernel/dumpstack.c	2012-05-03 14:51:43.805197563 -0500
> @@ -186,7 +186,9 @@ void dump_stack(void)
>  {
>  	unsigned long bp;
>  	unsigned long stack;
> +	static DEFINE_SPINLOCK(lock);	/* Serialise the printks */
>  
> +	spin_lock(&lock);
>  	bp = stack_frame(current, NULL);
>  	printk("Pid: %d, comm: %.20s %s %s %.*s\n",
>  		current->pid, current->comm, print_tainted(),
> @@ -194,6 +196,7 @@ void dump_stack(void)
>  		(int)strcspn(init_utsname()->version, " "),
>  		init_utsname()->version);
>  	show_trace(NULL, NULL, &stack, bp);
> +	spin_unlock(&lock);
>  }
>  EXPORT_SYMBOL(dump_stack);
>  
> -- 
> Russ Anderson, OS RAS/Partitioning Project Lead  
> SGI - Silicon Graphics Inc          rja@....com
> --
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