Re: URCU feature request?

["Paul E. McKenney via lttng-dev" <lttng-dev@lists.lttng.org>]

On Tue, Sep 02, 2025 at 04:24:16PM +0200, Thobias Knudsen wrote:
> Figured out what rcu_read_ongoing does. It just returns true if it's called
> within a read section and false otherwise. The problem for catching whether
> reads are done outside read sections is that you can not make macros for
> read and write operations. e.g. ptr->a = 4;.

We usually rely on the debugging code in rcu_dereference() to check this
sort of thing.  Or are you worried about the user leaking RCU-protected
pointers out past the rcu_read_unlock()?

							Thanx, Paul

> tir. 2. sep. 2025 kl. 16:17 skrev Thobias Knudsen <thobknu@gmail.com>:
> 
> > > 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
> >>
> >