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

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Message-ID: <20170928084535.GA19060@arm.com>
Date:   Thu, 28 Sep 2017 09:45:35 +0100
From:   Will Deacon <will.deacon@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Thu, Sep 28, 2017 at 10:38:01AM +0200, Peter Zijlstra wrote:
> 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' :-)

I know this sounds daft, but I think one of the big reasons why
lockless_dereference() doesn't get an awful lot of use is because it's
such a mouthful! Why don't we just move the smp_read_barrier_depends()
into READ_ONCE? Would anybody actually care about the potential impact on
Alpha (which, frankly, is treading on thin ice given the low adoption of
lockless_dereference())?

> 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.

Yeah, I'd be happy to help with that as a separate series. I already tripped
over 5 or 6 page table walkers in arch/arm64/ alone :(

Will