Re: [PATCH 07/13] mm/gup: Fix the lockless PMD access

[Peter Zijlstra]

On Sat, Oct 22, 2022 at 05:42:18PM -0700, Hugh Dickins wrote:
> On Sat, 22 Oct 2022, Peter Zijlstra wrote:
> 
> > On architectures where the PTE/PMD is larger than the native word size
> > (i386-PAE for example), READ_ONCE() can do the wrong thing. Use
> > pmdp_get_lockless() just like we use ptep_get_lockless().
> 
> I thought that was something Will Deacon put a lot of effort
> into handling around 5.8 and 5.9: see "strong prevailing wind" in
> include/asm-generic/rwonce.h, formerly in include/linux/compiler.h.
> 
> Was it too optimistic?  Did the wind drop?
> 
> I'm interested in the answer, but I've certainly no objection
> to making this all more obviously robust - thanks.

READ_ONCE() can't do what the hardware can't do. There is absolutely no
way i386 can do an atomic 64bit load without resorting to cmpxchg8b.

Also see the comment that goes with compiletime_assert_rwonce_type(). It
explicitly allows 64bit because there's just too much stuff that does
that (and there's actually 32bit hardware that *can* do it).

But it's still very wrong.