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

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

----- On Jul 13, 2020, at 2:46 PM, Olivier Dion olivier.dion@polymtl.ca wrote:

> On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Jul 13, 2020, at 11:19 AM, Olivier Dion olivier.dion@polymtl.ca wrote:
>>
>>> On Mon, 13 Jul 2020, Mathieu Desnoyers <mathieu.desnoyers@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