On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoy...@efficios.com> wrote:
> ----- 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 ?
I assumed __tracepoints_init() initialized rcu, but apparently __ptrs do
the same and more.
>
>> 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 ?
I assume here that the desired trace provider is part of a share object
that has already been dlopen() before.
Using RTLD_DEFAULT or simply NULL should find the correct symbol in the
executable if the share object that has the trace provider is _already_
loaded in memory.
Otherwise, the macro should be something like
'tracepoint_acquire(provider, so_path)' I guess? And so this would
indeed require a dlopen() on so_path and so on.
>
>> 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 (?)
Yes of course!
>
>> #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 ?
__tracepoints__init() is statically defined in every compilation units
that include the trace header. So this one doesn't actually need to be
externally visible, my mistake. Although I don't understand why this
initializer is duplicated across units.
However, __tracepoints__ptrs__init() is statically defined in one
compilation unit, the unit that has defined the TRACEPOINT_DEFINE macro.
So I guess that the pointer tables is unique for every exe/so. If
that's the case, then this initializer should also be find with dlsym()?
--
Olivier Dion
PolyMtl
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