On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> ----- On Jul 13, 2020, at 11:19 AM, Olivier Dion olivier.d...@polymtl.ca
> wrote:
>
>> On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoy...@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
_______________________________________________
lttng-dev mailing list
lttng-dev@lists.lttng.org
https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
