Re: [PATCH lttng-ust] Add ctor/dtor priorities for tracepoints/events

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

----- On Jul 12, 2020, at 11:49 AM, Olivier Dion olivier.dion@polymtl.ca wrote:

> On Sun, 12 Jul 2020, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Jul 11, 2020, at 11:29 AM, lttng-dev lttng-dev@lists.lttng.org wrote:
>>
>>> Some library might want to generate events in their ctor/dtor.  If
>>> LTTng initialize/finalize its tracepoints/events at the wrong time,
>>> events are lost.
>>> 
>>> Order of execution of the ctor/dtor is determined by priority.  When
>>> some priorities are equal, the order of execution seems to be
>>> determined by:
>>> 
>>>	   a) Order of appearance if in the same compilation unit
>>> 
>>>	   b) Order of link if in different compilation units
>>> 
>>>	   c) Order of load by ld-linux.so or dlopen(3) for
>>>	      share objects
>>
>> I recall different rules about constructor priorities. Can you provide
>> links to documentation stating the priority order you describe above ?
> 
> I haven't found any documentation on that.  This is purely empirical.
> Although I'm sure that we can dig something if chatting on GCC's IRC.

If it is not documented, then I am reluctant on depending on a behavior
which may be what happens today, but may not be the same for past/future
toolchains.

> 
>> Also, we should compare two approaches to fulfill your goal:
>> one alternative would be to have application/library constructors
>> explicitly call tracepoint constructors if they wish to use them.
> 
> I would prefer this way.  The former solution might not work in some
> cases (e.g. with LD_PRELOAD and priority =101) and I prefer explicit
> initialization in that case.
> 
> I don't see any cons for the second approach, except making the symbols
> table a few bytes larger.  I'll post a patch soon so we can compare and
> try to find more documentation on ctor priority.

And users will have to explicitly call the constructor on which they
depend, but I don't see it as a huge burden.

Beware though that there are a few configurations which can be used for
probe providers (see lttng-ust(3)).

Thanks,

Mathieu

> 
> --
> Olivier Dion
> PolyMtl

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com