linux-kernel - Re: [PATCH RT 2/2] ftrace: fix elevated preempt

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Date:	Tue, 19 Aug 2008 16:07:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH RT 2/2] ftrace: fix elevated preempt_count in
	wakeup-tracer

On Tue, 2008-08-19 at 09:45 -0400, Steven Rostedt wrote:
> On Tue, 19 Aug 2008, Gregory Haskins wrote:
> > > 
> > > Is preempt_count > 1 at all times here?
> > > 
> > > If not, it might drop to 0 and any interrupt might cause preemption -
> > > and its not obvious to me that that is actually correct.
> > >   
> > According to Steve, we are already in an interrupt-disabled section here, but
> > I will defer to him.  He suggested I try this over an IRC conversation when I
> > noticed a strange wakeup trace, and it seems to have solved the problem.
> 
> Exactly.
> 
> Peter,
> 
> We disable preemption, do a per_cpu check, if we are not nested we grab 
> disable interrupts and grab the spin lock.
> 
> The issue that Gregory brought up, was that the tracer would always show 
> that preemption was disabled, when it really wasn't. It was recording the
> preempt disabled that the trace function itself made. The answer was 
> simply to do as Greg did, and enable preemption (but interrupts are 
> still disabled), call the function tracer, then disable preemption again 
> (to match the outside preemption enabling.

Ok good. 

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