linux-kernel - Re: [PATCH v4 07/11] READ_ONCE: Enforce atomicity for {READ,WRITE}

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Message-ID: <20200424174334.GB13592@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Apr 2020 19:43:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     Jann Horn <jannh@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Mark Rutland <mark.rutland@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v4 07/11] READ_ONCE: Enforce atomicity for
 {READ,WRITE}_ONCE() memory accesses

On Fri, Apr 24, 2020 at 06:11:35PM +0100, Will Deacon wrote:
> My apologies, you're completely right. I thought that PAE mandated 64-bit
> atomicity, like it does on 32-bit ARM, but that's apparently not the case
> and looking at the 32-bit x86 pgtable code they have to be really careful
> there.

They added CMPXCHG8B for PAE, but then we never used it for that because
it's dead slow (MS did use it I think). Instead we play horrible split
word games, grep for CONFIG_GUP_GET_PTE_LOW_HIGH.