Re: [PATCH 1/2] rcu: Fix casting while dereferencing rcu pointers

[David Laight]

On Wed, 26 Aug 2026 13:28:42 +0200
Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:

> Hi David
> 
> On Wed, 26 Aug 2026 at 11:55, David Laight <david.laight.linux@xxxxxxxxx> wrote:
> >
> > On Tue, 25 Aug 2026 20:02:10 +0000
> > Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:  
> > >
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index 44c07a66edff..80276cedda80 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -488,7 +488,7 @@ static __always_inline bool lockdep_assert_rcu_helper(bool c, const struct __ctx
> > >  context_unsafe(                                                              \
> > >       typeof(*p) *local = (typeof(*p) *__force)(p);                   \
> > >       rcu_check_sparse(p, __rcu);                                     \
> > > -     ((typeof(*p) __force __kernel *)(local))                        \
> > > +     ((TYPEOF_UNQUAL(*p) __force __kernel *)(local))                 \  
> >
> > Ugg... TYPEOF_UNQUAL() is absolutely horrid...
> > When __CHECKER__ is undefined the whole thing is just (p).  
> 
> About using __CHECKER__: Are you suggesting using #ifdef __CHECKER__
> to use TYPEOF_UNQUAL or the current code?
> 
> I am open to doing that, but I would like to hear from Paul if he
> favors adding the (also horrid) #ifdef to save some CPU cycles on
> older compilers.
> 
> >
> > I suspect reducing all the definitions would measurably improve
> > kernel compile time (even before these changes).  
> 
> When I checked earlier in the other patch in this patchset, with my
> compiler I can hardly see any difference in compilation times [1].
> This is probably because GCC15 direclty maps TYPEOF_UNQUAL to the
> built-in __typeof_unqual__.

I was thinking of the _Generic() version.

But here you only want to remove the (I think) __user marker.
I don't think you want to get rid of 'const'.
So you don't really want gcc to remove any qualifiers.

There is an similar(ish) issue when an __force cast is used to
remove one of the extra qualifiers.
You really want the compiler to ignore the cast - so it checks
the actual types match.

David

> 
> 
> Thanks!
> 
> 
> [1] https://lore.kernel.org/all/CANiDSCsMrULsrGQg76x28GRXLg+HSKFWhhA14Zcn8hdm4KyOVw@xxxxxxxxxxxxxx/
> 
> >
> > David