Re: [PATCHv2 05/17] nvme: add Clang context annotations for nvme_ns_head::current_path

[Paul E. McKenney]

On Sat, Jun 27, 2026 at 05:38:44PM +0200, Marco Elver wrote:
> On Fri, 26 Jun 2026 at 20:36, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > On Fri, Jun 26, 2026 at 08:40:50AM +0200, Christoph Hellwig wrote:
> > > On Sun, Jun 14, 2026 at 06:45:20PM +0530, Nilay Shroff wrote:
> > > > +++ b/drivers/nvme/host/multipath.c
> > > > @@ -231,8 +231,16 @@ bool nvme_mpath_clear_current_path(struct nvme_ns *ns)
> > > >     bool changed = false;
> > > >     int node;
> > > >
> > > > +   /*
> > > > +    * This helper is used by namespace failover/teardown and I/O policy
> > > > +    * update paths. We only compare the head->current_path[] pointer value
> > > > +    * and do not dereference the referenced namespace, so suppress the
> > > > +    * context analysis warning for this lockless inspection of the
> > > > +    * __rcu_guarded pointer.
> > > > +    */
> > > >     for_each_node(node) {
> > > > -           if (ns == rcu_access_pointer(head->current_path[node])) {
> > > > +           if (context_unsafe(ns ==
> > > > +                           rcu_access_pointer(head->current_path[node]))) {
> > >
> > > I think we need a helper for this, as for a simple pointer value
> > > comparison without a dereference we don't really need either
> > > rcu_access_pointer nor locking.
> > >
> > > Maybe somthing like a
> > >
> > >       rcu_compare_pointer(rcu_pointer, nonrcu_pointer)
> > >
> > > ?
> >
> > We could provide something like this:
> >
> > #define rcu_compare_pointer(rcu_pointer, nonrcu_pointer) \
> >         context_unsafe(rcu_access_pointer(rcu_pointer) == (nonrcu_pointer))
> >
> > Or maybe rcu_pointer_equals()?
> >
> > Marco, thoughts?
> 
> I see 2 options:
> 
> 1. rcu_compare_pointer / rcu_pointer_equals as proposed. It's more
> explicit but does add some friction during Context Analysis
> enablement. But this only makes sense if there are cases where
> rcu_access_pointer() should be used under the RCU reader lock, which
> led me to the next suggestion...
> 
> 2. Redefine rcu_access_pointer() to just not require the RCU read-side
> lock to be held as:
> 
> > #define rcu_access_pointer(p) context_unsafe(__rcu_access_pointer((p), __UNIQUE_ID(rcu), __rcu))
> 
> [ Or alternatively wrap __rcu_access_pointer() in context_unsafe()
>   (similar to rcu_assign_pointer and friends). ]
> 
> I think rcu_access_pointer() is in the same category as the other RCU
> pointer helpers, although currently only the pointer update helpers
> imply context_unsafe(); I think it might have been an oversight
> initially, and we should change rcu_access_pointer() to match. There
> should be no reason why an rcu_access_pointer() should be protected by
> the RCU read-side critical section, since it's not meant for
> dereferencing the pointer (that'd be a bug; its documentation also
> says "Return the value of the specified RCU-protected pointer, but
> omit the lockdep checks for being in an RCU read-side critical section
> [...]").
> 
> If you agree there should be no cases where an rcu_access_pointer()
> should be guarded by an RCU read-side critical section, then I think
> this is the simplest and correct design, and avoids expanding the RCU
> API.

I don't know of any uses of rcu_access_pointer() within an RCU read-side
critical section, but code in a function that might be called both within
and outside of a critical section might well use rcu_access_pointer().
In other words, it should be OK to use rcu_access_pointer() within an
RCU read-side critical section as well as outside of one.

Or am I missing the point of your question?

							Thanx, Paul