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

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Date:   Fri, 25 Nov 2016 17:49:45 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     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 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). And the mm/ code is perfectly fine with these
PTE accesses being done NOT atomic.


> 
> The advantage of introducing the SINGLE_{LOAD,STORE}() helpers is that
> they compiletime validate this the size is 'right' and can runtime check
> alignment constraints.
> 
> IE, they are strictly stronger than {READ,WRITE}_ONCE().
>