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Date: Thu, 15 Mar 2018 07:52:23 -0400 (EDT)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>, rostedt <rostedt@goodmis.org>,
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Subject: Re: WARNING in tracepoint_probe_register_prio (2)
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----- On Mar 15, 2018, at 5:19 AM, Jiri Olsa jolsa@redhat.com wrote:

> On Thu, Mar 15, 2018 at 09:31:25AM +0100, Peter Zijlstra wrote:
>> On Wed, Mar 14, 2018 at 05:37:46PM -0400, Steven Rostedt wrote:
>> > On Tue, 13 Mar 2018 11:29:51 -0400 (EDT)
>> > Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>> > 
>> > > Here is a WARN_ON() splat in tracepoint.c, which I suspect is caused
>> > > by perf trying to register the same probe twice to the tracepoint API.
>> > > We got another splat on unregister too, which I will forward in a
>> > > separate email.
>> > > 
>> > > Thoughts ?
>> > 
>> > Yes, it looks like it's perf not accounting for registered events
>> > properly.
>> > 
>> > Peter?
>> 
>> I've not yet managed to reproduce, but if you look at the provided
>> repro.c file, you'll see it opens two _different_ events.
> 
> from the log it looks like they inject the slab error,
> and the allocation fails.. looks like we need to change
> the WARN to skip ENOMEM.. something like below?

Oh, I missed this important point. Then we should only
warn if !-ENOMEM for both tracepoint_add_func and tracepoint_remove_func,
because each performs memory allocation under the hood.
Like the following:

--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -207,7 +207,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
                        lockdep_is_held(&tracepoints_mutex));
        old = func_add(&tp_funcs, func, prio);
        if (IS_ERR(old)) {
-               WARN_ON_ONCE(1);
+               WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);
                return PTR_ERR(old);
        }
 
@@ -239,7 +239,7 @@ static int tracepoint_remove_func(struct tracepoint *tp,
                        lockdep_is_held(&tracepoints_mutex));
        old = func_remove(&tp_funcs, func);
        if (IS_ERR(old)) {
-               WARN_ON_ONCE(1);
+               WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);
                return PTR_ERR(old);
        }
 
-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com