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

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Date:   Mon, 24 Oct 2022 09:42:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Will Deacon <will@...nel.org>, x86@...nel.org, willy@...radead.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de, ubizjak@...il.com
Subject: Re: [PATCH 07/13] mm/gup: Fix the lockless PMD access

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.