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

On Thu, Jul 2, 2020 at 1:18 PM Will Deacon [off-list ref] wrote:

On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote:

quoted

On Thu, Jul 2, 2020 at 11:48 AM Will Deacon [off-list ref] wrote:

quoted

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