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

[Olivier Dion]

On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
> ----- On Jul 13, 2020, at 11:19 AM, Olivier Dion olivier.dion at polymtl.ca wrote:
>
>> On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote:
> [...]
>> 
>>>>> 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.
>> 
>> The burden is small indeed.  But users should pay close attention to
>> release the references in a destructor too.
>> 
>>> Beware though that there are a few configurations which can be used for
>>> probe providers (see lttng-ust(3)).
>> 
>> I'm not following you here.  I don't see any configuration for provider
>> except TRACEPOINT_LOGLEVEL.  What should I be aware of?
>
> See sections "Statically linking the tracepoint provider" and
> "Dynamically loading the tracepoint provider" from lttng-ust(3). It's
> especially the dynamic loading I am concerned about, because then it
> becomes tricky for an instrumented .so (or app) to call the probe provider's
> constructor without dlopening it beforehand, because there are no dependencies
> from the instrumented module on probe symbols. And given you plan to call
> this from a constructor, it means the dynamic loader lock is already held,
> so even if we dlopen the probe provider from the instrumented constructor,
> I am not sure the dlopen'd .so's constructor will be allowed to run
> immediately.
>
> Maybe one thing that could work for the dynamic loading case would be to:
>
> - let the instrumented constructor dlopen its probe,
> - from the instrumented constructor, use dlsym to get the probe's constructor
>   symbols.
> - call those constructors.
>
> If this is common enough, maybe we would want to provide helpers for
> this.

Okay so to be clear.  __tracepoints__init() should be call first, then
__tracepoints__ptrs_init() and then dlsym(3) on
__lttng_events_init__provider() _if_ TRACEPOINT_PROBE_DYNAMIC_LINKAGE.

Reverse the steps in destructor.

And so would something along these lines work?
--------------------------------------------------------------------------------
#ifdef TRACEPOINT_PROBE_DYNAMIC_LINKAGE

#  define tracepoint_acquire(provider)                           \
        do {                                                     \
                void (*init)(void);                              \
                __tracepoints__init();                           \
                __tracepoints__ptrs_init();                      \
                init = dlsym(RTLD_DEFAULT,                       \
                             "__lttng_events_init__" #provider); \
                if (init) {                                      \
                        init();                                  \
                }                                                \
        } while(0)

#else

#  define tracepoint_acquire(provider)                                 \
        do {                                                           \
                __tracepoint__init();                                  \
                __tracepoints_ptrs_init();                             \
                _TP_COMBINE_TOKENS(__lttng_events_init__, provider)(); \
        } while(0)

#endif
--------------------------------------------------------------------------------

And then:
----------------------------------------------------------------------
#include "my-trace.h"

__attribute__((constructor))
static void my_ctor(void)
{
        tracepoint_acquire(my_provider);
        tracepoint(my_provider, my_event, my_state);
}

__attribute__((destructor))
static void my_ctor(void)
{
        tracepoint_release(my_provider)
}
----------------------------------------------------------------------

Of course, this requires making __tracepoints__* externally visibile.

-- 
Olivier Dion
PolyMtl