linux-kernel - Re: [PATCH 10/21] tracing/probe: Split trace_event related data from trace

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Message-ID: <20190618122322.6875b643@gandalf.local.home>
Date:   Tue, 18 Jun 2019 12:23:22 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 10/21] tracing/probe: Split trace_event related data
 from trace_probe

On Wed, 19 Jun 2019 01:14:09 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Mon, 17 Jun 2019 21:56:43 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 

> > > +static nokprobe_inline struct trace_kprobe *
> > > +trace_kprobe_primary_from_call(struct trace_event_call *call)
> > > +{
> > > +	struct trace_probe *tp = trace_probe_primary_from_call(call);
> > > +
> > > +	return container_of(tp, struct trace_kprobe, tp);  
> > 
> > 
> > Hmm, is there a possibility that trace_probe_primary_from_call() may
> > not have a primary?  
> 
> Good question! Of course if given event_call is not a kprobe event,
> it doesn't have primary (or any) trace_probe. But that must not happen
> unless user misuses it.
> And that list never be the empty, when the last trace probe is released,
> the event_call also unregistered and released. See unregister_trace_kprobe()
> for details. If there is no siblings on the list, the event_call is also
> unregistered before unregistering kprobes, and after unregistering kprobes
> the list is unlinked.
>  (Note that unregister_kprobe() will wait a quiescence period
> before return. This means all probe handlers are done before that.)

Yeah, I thought something like that. But perhaps the
trace_probe_primary_from_call() code should add a WARN_ON() is the list
is empty.

> 


> > >  
> > > -	ret = __enable_trace_kprobe(tk);
> > > -	if (ret) {
> > > +	enabled = false;
> > > +	list_for_each_entry(pos, trace_probe_probe_list(tp), list) {
> > > +		tk = container_of(pos, struct trace_kprobe, tp);
> > > +		ecode = __enable_trace_kprobe(tk);
> > > +		if (ecode)
> > > +			ret = ecode;	/* Save the last error code */
> > > +		else
> > > +			enabled = true;  
> > 
> > So, if we have some enabled but return an error code, what should a
> > caller think of that? Wouldn't it be an inconsistent state?  
> 
> Oops, good catch!
> This part is related to caller (ftrace/perf) so should be more careful.
> Usually, kprobe enablement should not fail. If one of them has
> gone (like a probe on unloaded module), it can be fail but that
> should be ignored. I would like to add some additional check so that
> - If all kprobes are on the module which is unloaded, enablement
>   must be failed and return error.
> - If any kprobe is enabled, and others are on non-exist modules,
>   it should succeeded and return OK.
> - If any kprobe caused an error not because of unloaded module,
>   all other enablement should be canceled and return error.
> 
> Is that OK for you?
> 

Sounds good to me.

-- Steve