Re: [PATCH v4 14/35] rcu: Support Clang's context analysis

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Dec 10, 2025 at 10:50:11PM +0100, Marco Elver wrote:
> On Wed, 10 Dec 2025 at 20:30, Paul E. McKenney <paulmck@kernel.org> wrote:
> > On Thu, Nov 20, 2025 at 04:09:39PM +0100, Marco Elver wrote:
> > > Improve the existing annotations to properly support Clang's context
> > > analysis.
> > >
> > > The old annotations distinguished between RCU, RCU_BH, and RCU_SCHED;
> > > however, to more easily be able to express that "hold the RCU read lock"
> > > without caring if the normal, _bh(), or _sched() variant was used we'd
> > > have to remove the distinction of the latter variants: change the _bh()
> > > and _sched() variants to also acquire "RCU".
> > >
> > > When (and if) we introduce context guards to denote more generally that
> > > "IRQ", "BH", "PREEMPT" contexts are disabled, it would make sense to
> > > acquire these instead of RCU_BH and RCU_SCHED respectively.
> 
>  ^

"I can't read!"  ;-)

> > > The above change also simplified introducing __guarded_by support, where
> > > only the "RCU" context guard needs to be held: introduce __rcu_guarded,
> > > where Clang's context analysis warns if a pointer is dereferenced
> > > without any of the RCU locks held, or updated without the appropriate
> > > helpers.
> > >
> > > The primitives rcu_assign_pointer() and friends are wrapped with
> > > context_unsafe(), which enforces using them to update RCU-protected
> > > pointers marked with __rcu_guarded.
> > >
> > > Signed-off-by: Marco Elver <elver@google.com>
> >
> > Good reminder!  I had lost track of this series.
> >
> > My big questions here are:
> >
> > o       What about RCU readers using (say) preempt_disable() instead
> >         of rcu_read_lock_sched()?
> 
> The infrastructure that is being built up in this series will be able
> to support this, it's "just" a matter of enhancing our various
> interfaces/macros to use the right annotations, and working out which
> kinds of contexts we want to support. There are the obvious
> candidates, which this series is being applied to, as a starting
> point, but longer-term there are other kinds of context rules that can
> be checked with this context analysis. However, I think we have to
> start somewhere.
> 
> > o       What about RCU readers using local_bh_disable() instead of
> >         rcu_read_lock_sched()?
> 
> Same as above; this requires adding the necessary annotations to the
> BH-disabling/enabling primitives.
> 
> > And keeping in mind that such readers might start in assembly language.
> 
> We can handle this by annotating the C functions invoked from assembly
> with attributes like  __must_hold_shared(RCU) or
> __releases_shared(RCU) (if the callee is expected to release the RCU
> read lock / re-enable preemption / etc.) or similar.
> 
> > One reasonable approach is to require such readers to use something like
> > rcu_dereference_all() or rcu_dereference_all_check(), which could then
> > have special dispensation to instead rely on run-time checks.
> 
> Agree. The current infrastructure encourages run-time checks where the
> static analysis cannot be helped sufficiently otherwise (see patch:
> "lockdep: Annotate lockdep assertions for context analysis").

OK, very good.

> > Another more powerful approach would be to make this facility also
> > track preemption, interrupt, NMI, and BH contexts.
> >
> > Either way could be a significant improvement over what we have now.
> >
> > Thoughts?
> 
> The current infrastructure is powerful enough to allow for tracking
> more contexts, such as interrupt, NMI, and BH contexts, and as I
> hinted above, would be nice to eventually get to!  But I think this is
> also a question of how much do we want to front-load for this to be
> useful, and what should incrementally be enhanced while the baseline
> infrastructure is already available.
> 
> I think the current series is the baseline required support to be
> useful to a large fraction of "normal" code in the kernel.

Makes sense to me!

> On a whole, my strategy was to get to a point where maintainers and
> developers can start using context analysis where appropriate, but at
> the same time build up and incrementally add more supported contexts
> in parallel. There's also a good chance that, once baseline support
> lands, more interested parties contribute and things progress faster
> (or so I'd hope :-)).

I know that feelling!  ;-)

OK, for this patch and the SRCU patch based on a quick once-over:

Acked-by: Paul E. McKenney <paulmck@kernel.org>

							Thanx, Paul