linux-kernel - Re: [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar

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Message-ID: <20200113150151.GC4458@willie-the-truck>
Date:   Mon, 13 Jan 2020 15:01:52 +0000
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar_typeof'
 for load-acquire macros

On Fri, Jan 10, 2020 at 08:42:37PM +0100, Arnd Bergmann wrote:
> On Fri, Jan 10, 2020 at 5:57 PM Will Deacon <will@...nel.org> wrote:
> 
> > @@ -128,10 +128,10 @@ do {                                                                      \
> >  #ifndef __smp_load_acquire
> >  #define __smp_load_acquire(p)                                          \
> >  ({                                                                     \
> > -       typeof(*p) ___p1 = READ_ONCE(*p);                               \
> > +       __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);               \
> >         compiletime_assert_atomic_type(*p);                             \
> >         __smp_mb();                                                     \
> > -       ___p1;                                                          \
> > +       (typeof(*p))___p1;                                              \
> >  })
> 
> Doesn't that last  (typeof(*p))___p1 mean you put the potential
> 'volatile' back on the assignment after you went through the
> effort of taking it out?

Yes, but that's ok wrt codegen since the local variable isn't volatile,
and I definitely ran into issues with 'const' if I returned the unqualified
type here.

Will