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

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Message-ID: <55EC4DD7.80208@oracle.com>
Date:	Sun, 06 Sep 2015 10:29:43 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Minfei Huang <mhuang@...hat.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 09/06/2015 05:20 AM, Minfei Huang wrote:
> 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.

Can you point me to exactly where trace_init_global_iter() initializes
iter->started?


Thanks,
Sasha
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