lkml.org 
[lkml]   [2016]   [Nov]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
From
Date
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().
>

\
 
 \ /
  Last update: 2016-11-25 17:51    [W:0.356 / U:0.156 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site