netdev - Re: [PATCH 0/3] virtio/vringh: kill off ACCESS

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Message-ID: <20161125172624.GA30811@leverpostej>
Date:   Fri, 25 Nov 2016 17:28:01 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>, dbueso@...e.de,
        jasowang@...hat.com, KVM list <kvm@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

On Fri, Nov 25, 2016 at 05:49:45PM +0100, Christian Borntraeger wrote:
> On 11/25/2016 05:17 PM, Peter Zijlstra wrote:
> > On Fri, Nov 25, 2016 at 04:10:04PM +0000, Mark Rutland wrote:
> >> On Fri, Nov 25, 2016 at 04:21:39PM +0100, Dmitry Vyukov wrote:
> > 
> >>> What are use cases for such primitive that won't be OK with "read once
> >>> _and_ atomically"?
> >>
> >> I have none to hand.
> > 
> > Whatever triggers the __builtin_memcpy() paths, and even the size==8
> > paths on 32bit.
> > 
> > You could put a WARN in there to easily find them.
> 
> There were several cases that I found during writing the *ONCE stuff.
> For example there are some 32bit ppc variants with 64bit PTEs. Some for
> others (I think sparc).

We have similar on 32-bit ARM w/ LPAE. LPAE implies that a naturally
aligned 64-bit access is single-copy atomic, which is what makes that
ok.

> And the mm/ code is perfectly fine with these PTE accesses being done
> NOT atomic.

That strikes me as surprising. Is there some mutual exclusion that
prevents writes from occuring wherever a READ_ONCE() happens to a PTE?

Otherwise, how is tearing not a problem? Does it have some pattern like
the lockref cmpxchg?

Thanks,
Mark.