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

[Arnd Bergmann]

On Thu, Jul 2, 2020 at 1:18 PM Will Deacon <will@xxxxxxxxxx> wrote:
> On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote:
> > 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:

> Not sure I follow you here, but I can confirm that what you're worried
> about doesn't happen for the usual case of a pointer-to-volatile scalar.
>
> For example, ignoring dependency ordering:
>
> unsigned long foo(volatile unsigned long *p)
> {
>         return smp_load_acquire(p) + 1;
> }
>
> Ends up looking like:
>
>         unsigned long ___p1 = *(const volatile unsigned long *)p;
>         smp_mb();
>         (volatile unsigned long)___p1;
>
> My understanding is that casting a non-pointer type to volatile doesn't
> do anything, so we're good.

Right, I mixed up the correct

        (typeof(*p))___p;

with the incorrect

       *typeof(p)&___p;

which would dereference a volatile pointer and cause the
problem.

The code is all fine then.

    Arnd