Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2016-11-25 21:09:02
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kvm, lkml, virtualization
On Fri, Nov 25, 2016 at 05:49:45PM +0100, Christian Borntraeger wrote:
On 11/25/2016 05:17 PM, Peter Zijlstra wrote:quoted
On Fri, Nov 25, 2016 at 04:10:04PM +0000, Mark Rutland wrote:quoted
On Fri, Nov 25, 2016 at 04:21:39PM +0100, Dmitry Vyukov wrote:quoted
quoted
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.
In that case do we even need _ONCE at all? Are there assumptions these are two 32 bit reads?
quoted
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().