Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation

[Arnd Bergmann]

On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will@xxxxxxxxxx> wrote:
> On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:
> > On Tue, Jun 30, 2020 at 06:37:20PM +0100, Will Deacon wrote:
> > > -#define read_barrier_depends() __asm__ __volatile__("mb": : :"memory")
> > > +#define __smp_load_acquire(p)                                              \
> > > +({                                                                 \
> > > +   __unqual_scalar_typeof(*p) ___p1 =                              \
> > > +           (*(volatile typeof(___p1) *)(p));                       \
> > > +   compiletime_assert_atomic_type(*p);                             \
> > > +   ___p1;                                                          \
> > > +})
> >
> > Sorry if I'm being thick, but doesn't this need a barrier after the
> > volatile access to provide the acquire semantic?
> >
> > IIUC prior to this commit alpha would have used the asm-generic
> > __smp_load_acquire, i.e.
> >
> > | #ifndef __smp_load_acquire
> > | #define __smp_load_acquire(p)                                           \
> > | ({                                                                      \
> > |         __unqual_scalar_typeof(*p) ___p1 = READ_ONCE(*p);               \
> > |         compiletime_assert_atomic_type(*p);                             \
> > |         __smp_mb();                                                     \
> > |         (typeof(*p))___p1;                                              \
> > | })
> > | #endif

I also have a question that I didn't dare ask when the same
code came up before (I guess it's also what's in the kernel today):

With the cast to 'typeof(*p)' at the end, doesn't that mean we
lose the effect of __unqual_scalar_typeof() again, so any "volatile"
pointer passed into __READ_ONCE_SCALAR() or
__smp_load_acquire() still leads to a volatile load of the original
variable, plus another volatile access to ___p1 after
spilling it to the stack as a non-volatile variable?

I hope I'm missing something obvious here.

        Arnd