Re: [PATCH 02/26] rewrite READ_ONCE/WRITE_ONCE

Peter Zijlstra
2017-03-03T16:20:21Z

On Fri, Mar 03, 2017 at 03:49:38PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 03, 2017 at 09:26:50AM +0100, Christian Borntraeger wrote:
> > Right. The main purpose is to read/write _ONCE_. You can assume a somewhat
> > atomic access for sizes <= word size. And there are certainly places that
> > rely on that. But the *ONCE thing is mostly used for things where we used
> > barrier() 10 years ago.
>
> A lot of code relies on READ/WRITE_ONCE() to generate single
> instructions for naturally aligned machined word sized loads/stores
> (something GCC used to guarantee, but does no longer IIRC).
>
> So much so that I would say its a bug if READ/WRITE_ONCE() doesn't
> generate a single instruction under those conditions.
>
> However, every time I've tried to introduce stricter
> semantics/primitives to verify things Linus hated it.

See here for the last attempt: