linux-kernel - Re: [PATCH 1/4] ftrace: Fix function pid filter on instances

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Message-ID: <20170328222855.78af0d53@grimm.local.home>
Date:   Tue, 28 Mar 2017 22:28:55 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com
Subject: Re: [PATCH 1/4] ftrace: Fix function pid filter on instances

On Wed, 29 Mar 2017 11:20:37 +0900
Namhyung Kim <namhyung@...nel.org> wrote:


> > Actually, if this is called after event_trace_del_tracer(), the tr is
> > already invisible and nothing new should change.  
> 
> I don't follow.  After event_trace_del_tracer(), the tr is invisible
> from the probe of event tracing but still is visible from the probe of
> function tracing, right?

Well, nothing should be able to get to the set_ftrace_filter file when
there. Because of the tr->ref count. But keeping the lock is safer
regardless, and it's not a fast path, so the extra overhead if the lock
isn't needed is no big deal.

> 
> > 
> > Make a wrapper around clear_ftrace_pids() and call that instead. We
> > don't even need to take a lock, but as I see there's a lockdep test for
> > ftrace_lock, we should still do so just to be safe.  
> 
> Right, that's why I call ftrace_pid_reset() instead of
> clear_ftrace_pids().  So do you prefer adding a new wrapper like below
> rather than reusing ftrace_pid_reset() with a new argument?

Yes, because the bool passed in is confusing. A separate function like
below is more descriptive.

-- Steve

> 
> Thanks,
> Namhyung
> 
> 
> > 
> > void ftrace_clear_pids(struct trace_array *tr)
> > {
> > 	mutex_lock(&ftrace_lock);
> > 
> > 	clear_ftrace_pids(tr);
> > 
> > 	mutex_unlock(&ftrace_lock);
> > }