Re: URCU feature request?

[Thobias Knudsen via lttng-dev <lttng-dev@lists.lttng.org>]

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> I suspect that what you are trying to achieve is validation that
> calls to functions that require to be within a RCU read-side critical
> section such as cds_lfht_lookup are indeed invoked from within a
> critical section.

Yes exactly, but not only for read sections. It could also be to validate
that rcu_barrier isnt called within a callback function or that
rcu_quiescent_state isnt called when thread is offline

> We've added a "rcu_read_ongoing()" API for that purpose to liburcu
> RCU flavors, but AFAIK it's not used for any kind of validation
> within cds_lfht. This could indeed become a feature request that
> would apply to all liburcu data structure APIs.

I can't find any documentation on rcu_read_ongoing. Btw the documentation
for urcu in general is scattered all around the repo it seems. It should
have been all at one place imo. Should I make a feature request for it? I
don't know how stuff works inside this repo though.



man. 1. sep. 2025 kl. 17:09 skrev Mathieu Desnoyers <
mathieu.desnoyers@efficios.com>:

> On 2025-08-31 16:48, Thobias Knudsen wrote:
> > BTW i've made a macro library which overrides the
> > urcu and lfht functions. It checks that you call the functions in the
> > correct order. This has been really useful for debugging and I think it
> > would be useful to have it integrated into urcu to make the it more user
> > friendly. you could have it integrated with the macro DEBUG_RCU or make
> > a new one, maybe DEBUG_FUNCTION_CALL_ORDER. The only thing it doesn't
> > catch is if you continue to use data from cds_lfht_lookup() after
> > rcu_read_unlock()
>
> I suspect that what you are trying to achieve is validation that
> calls to functions that require to be within a RCU read-side critical
> section such as cds_lfht_lookup are indeed invoked from within a
> critical section.
>
> We've added a "rcu_read_ongoing()" API for that purpose to liburcu
> RCU flavors, but AFAIK it's not used for any kind of validation
> within cds_lfht. This could indeed become a feature request that
> would apply to all liburcu data structure APIs.
>
> Thanks,
>
> Mathieu
>
> >
> > søn. 31. aug. 2025 kl. 22:42 skrev Thobias Knudsen <thobknu@gmail.com
> > <mailto:thobknu@gmail.com>>:
> >
> >     First question: why isn't Userspace RCU more popular?
> >     Second question: Why isn't it possible to create issues in the urcu
> >     repo? Im asking for a feature here:
> >     Is it possible to make a function which checks if there are more
> >     callbacks queued by call_rcu? I need that because there are some
> >     callbacks which call call_rcu, and then calling rcu_barrier once
> >     isn't enough and I therefore need a way to check if there are still
> >     callbacks to wait on before terminating the program. No worries if
> >     this is too much to ask for or if it isn't possible. This
> >     functionality is only needed in termination because otherwise
> >     isn't needed to call rcu_barrier multiple times (at least so far)
> >
> >     Best regards
> >     Thobias
> >
> >
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>

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