On Wed, Aug 09, 2017 at 05:06:03PM +0200, Peter Zijlstra wrote:
> Now, ARM64 for instance plays funny games, it does something along the
> lines of:
>
> cmpxchg(ptr, old, new)
> {
> do {
> r = LL(ptr);
> if (r != old)
> return r; /* no barriers */
> r = new
> } while (SC_release(ptr, r));
> smp_mb();
> return r;
> }
>
> Thereby ordering things relative to the store on ptr, but the load can
> very much escape. The thinking is that if success, we must observe the
> latest value of ptr, but even in that case the load is not ordered and
> could happen before.
>
> However, since we're guaranteed to observe the latest value of ptr (on
> success) it doesn't matter if we reordered the load, there is no newer
> value possible.
>
> So heaps of tricky, but correct afaict. Will?
And could not PPC do something similar:
cmpxchg(ptr, old, new)
{
lwsync();
dp {
r = LL(ptr);
if (r != old)
return;
r = new;
} while (SC(ptr, r));
sync();
return r;
}
?
the lwsync would make it store-release on SC with similar reasoning as
above.
And lwsync allows 'stores reordered after loads', which allows the prior
smp_store_release() to leak past.
Or is the reason this doesn't work on PPC that its RCpc?
