Re: [PATCH RT 2/2] ftrace: fix elevated preempt_count in wakeup-tracer

[Gregory Haskins]

Peter Zijlstra wrote:
> On Tue, 2008-08-19 at 05:19 -0400, Gregory Haskins wrote:
>   
> > Suggested by Steve Rostedt to fix an observed "+1" in the preempt-count
> >
> > Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
> > ---
> >
> >  kernel/trace/trace_sched_wakeup.c |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/trace/trace_sched_wakeup.c b/kernel/trace/trace_sched_wakeup.c
> > index c3a15bd..ae523fd 100644
> > --- a/kernel/trace/trace_sched_wakeup.c
> > +++ b/kernel/trace/trace_sched_wakeup.c
> > @@ -70,7 +70,9 @@ wakeup_tracer_call(unsigned long ip, unsigned long parent_ip)
> >  	if (task_cpu(wakeup_task) != cpu)
> >  		goto unlock;
> >  
> > +	preempt_enable_no_resched_notrace();
> >  	trace_function(tr, data, ip, parent_ip, flags);
> > +	preempt_disable_notrace();
> >     
>
> 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.

Im really sending this patch more of a reminder to Steve that he was 
going to fix this, rather than to accept my patch as is.  Of course I 
don't mind if it is accepted as is, and I can make the prologue/comments 
more descriptive if necessary.  But if Steve wants to do something like 
fold this into his ftrace series, that is fine too.  I just didn't want 
it to be forgotten ;)

-Greg

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