[lttng-dev] LTTng-UST 2.0 mutex usage

[Mathieu Desnoyers]

* Oestman, Fredrik (Fredrik_Oestman at mentor.com) wrote:
> Hi,
> 
> 
> We're experimenting with tracing calls to the pthreads library using a
> LD_PRELOAD-ed interposer library, which is instrumented with UST
> tracepoints. In the startup phase, before the application has been
> called, probes are being registered, and one by one the tracepoints
> start emitting events. So some of the pthreads mutex motions of UST
> itself are being traced along.
> 
> What would be a good way of avoiding emitting events before the
> application itself has started, specifically events related to UST
> mutexes?

one way would be to implement mutexes within lttng-ust, so we would stop
using the pthread mutexes.

> The ust-ctl library appears to have a function ustctl_register_done(),
> is that useful?

No. This function is called by the session daemon when it has completed
sending all commands to the application so its internal lttng-ust state
matches that of the active tracing sessions.

We need to be aware that a lttng-ust library can interact with up to 2
session daemons: one for the user, one for root.

> (Is there an API documentation?) Or would it be better
> to find out which mutexes belong to UST and filter them out?

custom implementation of mutexes within lttng-ust sounds like the best
approach.

Thanks,

Mathieu

> By controlling our interposer library from within the application we
> can avoid the problem, but we'd rather do without recompiling the
> application, since it sort of would defeat the reason for using
> LD_PRELOAD in the first place.
> 
> 
> Cheers,
> 
> Fredrik Östman
> 
> 
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com