linux-kernel - Re: [PATCH] x86: Use preempt_disable_notrace() in cycles_2

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Date:	Tue, 4 Feb 2014 21:22:03 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: [PATCH] x86: Use preempt_disable_notrace() in cycles_2_ns()

On Tue, 4 Feb 2014, Steven Rostedt wrote:

> When debug preempt is enabled, preempt_disable() can be traced by
> function and function graph tracing.
> 
> There's a place in the function graph tracer that calls trace_clock()
> which eventually calls cycles_2_ns() outside of the recursion
> protection. When cycles_2_ns() calls preempt_disable() it gets traced
> and the graph tracer will go into a recursive loop causing a crash or
> worse, a triple fault.
> 
> Simple fix is to use preempt_disable_notrace() in cycles_2_ns, which
> makes sense because the preempt_disable() tracing may use that code
> too, and it tracing it, even with recursion protection is rather
> pointless.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by-me

> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> index 19e5adb..acb3b60 100644
> --- a/arch/x86/kernel/tsc.c
> +++ b/arch/x86/kernel/tsc.c
> @@ -209,7 +209,7 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
>  	 * dance when its actually needed.
>  	 */
>  
> -	preempt_disable();
> +	preempt_disable_notrace();
>  	data = this_cpu_read(cyc2ns.head);
>  	tail = this_cpu_read(cyc2ns.tail);
>  
> @@ -229,7 +229,7 @@ static inline unsigned long long cycles_2_ns(unsigned long long cyc)
>  		if (!--data->__count)
>  			this_cpu_write(cyc2ns.tail, data);
>  	}
> -	preempt_enable();
> +	preempt_enable_notrace();
>  
>  	return ns;
>  }
> 
--
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