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

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Date:	Tue, 4 Feb 2014 20:45:56 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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, Feb 04, 2014 at 02:13:15PM -0500, 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>

Indeed, thanks!
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