linux-kernel - Re: [PATCH] ftrace: allow dumping traces without tracking trace started cpus

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Date:	Sun, 6 Sep 2015 17:20:16 +0800
From:	Minfei Huang <mhuang@...hat.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	rostedt@...dmis.org, mingo@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: allow dumping traces without tracking trace
 started cpus

On 09/04/15 at 12:45pm, Sasha Levin wrote:
> We don't init iter->started when dumping the ftrace buffer, and there's no
> real need to do so - so allow skipping that check if the iter doesn't have
> an initialized ->started cpumask.
> 
> Signed-off-by: Sasha Levin <sasha.levin@...cle.com>
> ---
>  kernel/trace/trace.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 6e79408..0dc647f 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2671,13 +2671,14 @@ static void test_cpu_buff_start(struct trace_iterator *iter)
>  	if (!(iter->iter_flags & TRACE_FILE_ANNOTATE))
>  		return;
>  
> -	if (cpumask_test_cpu(iter->cpu, iter->started))
> +	if (iter->started && cpumask_test_cpu(iter->cpu, iter->started))
>  		return;

Ftrace will initialize the variable iter->started in the function
trace_init_global_iter. Otherwise kernel will panic during calling the
function cpumask_test_cpu.

So it is safe to call the function cpumask_test_cpu without doing
checking.

Thanks
Minfei
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