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

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Date:	Tue, 19 Aug 2008 15:12:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	mingo@...e.hu, rostedt@...dmis.org, 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 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@...ell.com>
> ---
> 
>  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.

Just asking, as neither the changelog nor the code fragment enlightens
me on the subject.

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