linux-kernel - Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE

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Message-ID: <20170928083801.m6rb4frbbgzgam2o@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Sep 2017 10:38:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     paulmck@...ux.vnet.ibm.com, kirill.shutemov@...ux.intel.com,
        linux-kernel@...r.kernel.org, ynorov@...iumnetworks.com,
        rruigrok@...eaurora.org, linux-arch@...r.kernel.org,
        akpm@...ux-foundation.org, catalin.marinas@....com
Subject: Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when
 accessing page tables

On Wed, Sep 27, 2017 at 04:49:28PM +0100, Will Deacon wrote:
> In many cases, page tables can be accessed concurrently by either another
> CPU (due to things like fast gup) or by the hardware page table walker
> itself, which may set access/dirty bits. In such cases, it is important
> to use READ_ONCE/WRITE_ONCE when accessing page table entries so that
> entries cannot be torn, merged or subject to apparent loss of coherence.

In fact, we should use lockless_dereference() for many of them. Yes
Alpha is the only one that cares about the difference between that and
READ_ONCE() and they do have the extra barrier, but if we're going to do
this, we might as well do it 'right' :-)

Also, a very long standing item on my TODO list is to see how much of it
we can unify across the various architectures, because there's a giant
amount of boiler plate involved with all this.