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

[Olivier Dion via lttng-dev <lttng-dev@lists.lttng.org>]

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.

> 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.

-- 
Olivier Dion
PolyMtl
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