----- On Jul 13, 2020, at 2:46 PM, Olivier Dion olivier.d...@polymtl.ca wrote:
> 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()
I don't think the order matters. What makes you think otherwise ?
> and then dlsym(3) on
> __lttng_events_init__provider() _if_ TRACEPOINT_PROBE_DYNAMIC_LINKAGE.
Yes.
>
> 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(); \
Where is the dlopen() done ? What code is responsible for it ?
> init = dlsym(RTLD_DEFAULT, \
This should use the handled returned by dlopen.
> "__lttng_events_init__" #provider); \
> if (init) { \
> init(); \
> } \
> } while(0)
>
We may want a dlclose on the release (?)
> #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.
Why is that so ?
Thanks,
Mathieu
>
> --
> Olivier Dion
> PolyMtl
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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