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

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Message-ID: <20170329030234.GD19448@sejong>
Date:   Wed, 29 Mar 2017 12:02:34 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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 Tue, Mar 28, 2017 at 10:58:43PM -0400, Steven Rostedt wrote:
> On Wed, 29 Mar 2017 11:42:27 +0900
> Namhyung Kim <namhyung@...nel.org> wrote:
> 
> > On Tue, Mar 28, 2017 at 10:28:55PM -0400, Steven Rostedt wrote:
> > > 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.  
> > 
> > Oh, I meant if a pid filter was already set when removing the
> > instance.  Function filters should be inactive since function tracer
> > was finished (via tracing_set_nop), but the probe on sched_switch
> > event (for pid filter) is still active and references the tr.
> > 
> 
> I think we are talking about two different things. I was simply talking
> about the need to take the ftrace_lock or not in the
> clear_ftrace_pids() call here. I don't think we have to, because nothing
> should be in contention with it at that point. But it doesn't hurt to
> take it.

Right, I agree with you wrt the locking.

Thanks,
Namhyung