Re: [RFC] LKMM: Add volatile_if()

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, Jun 04, 2021 at 05:42:28PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 04, 2021 at 05:22:04PM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 04, 2021 at 04:13:57PM +0100, Will Deacon wrote:
> > 
> > > In fact, maybe it's actually necessary to bundle the load and branch
> > > together. I looked at some of the examples of compilers breaking control
> > > dependencies from memory-barriers.txt and the "boolean short-circuit"
> > > example seems to defeat volatile_if:
> > > 
> > > void foo(int *x, int *y)
> > > {
> > >         volatile_if (READ_ONCE(*x) || 1 > 0)
> > >                 WRITE_ONCE(*y, 42);
> > > }  
> > 
> > Yeah, I'm not too bothered about this. Broken is broken.
> > 
> > If this were a compiler feature, the above would be a compile error. But
> > alas, we're not there yet :/ and the best we get to say at this point
> > is: don't do that then.
> 
> Ha! Fixed it for you:
> 
> #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))

Alan