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

[Hugh Dickins]

On Mon, 24 Oct 2022, Peter Zijlstra wrote:
> 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.

Right.

> 
> 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).

Yes, the "strong prevailing wind" comment. I think I've never read that
carefully enough, until you redirected me back there: it is in fact
quite clear, that it's only *atomic* in the Armv7 + LPAE case; but
READ_ONCEy (READ_EACH_HALF_ONCE I guess) for other 64-on-32 cases.

> 
> But it's still very wrong.

Somewhat clearer to me now, thanks.

Hugh