On 11/25/2016 05:17 PM, Peter Zijlstra wrote:

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.

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().