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: https://marc.info/?l=linux-virtualization&m=148007765918101&w=2
