Re: URCU feature request?

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

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> 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()?

Yes, that is what I'm worried about. For my case when using URCU that is my
biggest concern, as the debug macro "library" I made checks everything
else, if I haven't overlooked anything.

Thanks
Thobias

tir. 2. sep. 2025 kl. 22:33 skrev Paul E. McKenney <paulmck@kernel.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
> > >>
> > >
>

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