[lttng-dev] QSBR urcu question

[Mathieu Desnoyers]

----- On Apr 5, 2021, at 1:43 PM, lttng-dev lttng-dev at lists.lttng.org wrote:

> Hello all,
> 
> I am using the QSBR flavor of URCU and I have a question: is it ok to call
> rcu_register_thread more than once per thread?

Yes, but you should unregister before registering again.

> Similarly, is it ok to call
> rcu_unregister_thread more than once per thread?

Likewise: you should not have 2 unregistrations back-to-back without a
registration in between.

Note however that frequent registration/unregistration of a thread can lead to
significant performance overhead and lock contention on the rcu_registry_lock,
which is likely something you will want to avoid.

> I am using a library that uses
> a threadpool I have no control over. A thread from its threadpool invokes a
> callback function of mine. Threads may be reused. I would like to call
> rcu_register_thread at the beginning of my callback and rcu_unregister_thread
> at the end. The thread in question may exit or may be reused at some later
> point, I have no control over that. So I want to make sure that periodically
> registering and unregistering the same thread is ok. Thanks for any
> information.

You may want to use the "urcu-bp" flavor instead. It has a slightly higher
overhead than qsbr, but won't require as much fine-tuned integration with the
threadpool runtime.

But if you really want to do an efficient integration of urcu-qsbr with the threadpool
without changing any of the threadpool, here are a few ideas though:

- you could keep a "qsbr_registered" state in a TLS variable owned by your application,
- lazily perform qsbr registration if needed, based on the "qsbr_registered" state,
- put thread "offline" before returning control to the threadpool,
- put thread "online" when entering your code again,
- if the threadpool can call your application before the thread exits, then use that
  to unregister qsbr. Else, you could always use a dummy pthread key to have an application
  destructor called on thread exit, and unregister qsbr that way,

Hoping this help!

Best regards,

Mathieu

> 
> Jeff
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com