Re: [RFC] LKMM: Add volatile_if()

Alan Stern
2021-06-04T18:27:11Z

On Fri, Jun 04, 2021 at 06:17:20PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 11:51:54AM -0400, Alan Stern wrote:
> > On Fri, Jun 04, 2021 at 05:42:28PM +0200, Peter Zijlstra wrote:
>
> > > #define volatile_if(cond) if (({ bool __t = (cond); BUILD_BUG_ON(__builtin_constant_p(__t)); volatile_cond(__t); }))
> >
> > That won't help with more complicated examples, such as:
> >
> > volatile_if (READ_ONCE(*x) * 0 + READ_ONCE(*y))
>
> That's effectively:
>
> volatile_if (READ_ONCE(*y))
> WRITE_ONCE(*y, 42);

Sorry, what I meant to write was:

volatile_if (READ_ONCE(*x) * 0 + READ_ONCE(*y))
WRITE_ONCE(*z, 42);

where there is no ordering between *x and *z. It's not daft, and yes, a
macro won't be able to warn about it.

Alan

> which is a valid, but daft, LOAD->STORE order, no? A compiler might
> maybe be able to WARN on that, but that's definitely beyond what we can
> do with macros.