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

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Message-ID: <20170928154954.GA9892@arm.com>
Date:   Thu, 28 Sep 2017 16:49:54 +0100
From:   Will Deacon <will.deacon@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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, rth@...ddle.net, ink@...assic.park.msu.ru,
        mattst88@...il.com, linux-alpha@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] arm64: mm: Use READ_ONCE/WRITE_ONCE when
 accessing page tables

On Thu, Sep 28, 2017 at 08:43:54AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 28, 2017 at 09:45:35AM +0100, Will Deacon wrote:
> > 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())?
> 
> This is my cue to ask my usual question...  ;-)
> 
> Are people still running mainline kernels on Alpha?  (Added Alpha folks.)
> 
> As always, if anyone is, we must continue to support Alpha, but sounds
> like time to check again.

I'll be honest and say that I haven't updated mine for a while, but I do
have a soft spot for those machines :(

Will