Date: Thu, 28 Sep 2017 10:38:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: paulmck@linux.vnet.ibm.com, kirill.shutemov@linux.intel.com,
        linux-kernel@vger.kernel.org, ynorov@caviumnetworks.com,
        rruigrok@codeaurora.org, linux-arch@vger.kernel.org,
        akpm@linux-foundation.org, catalin.marinas@arm.com
Subject: Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when
 accessing page tables
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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.