Re: [RFC] LKMM: Add volatile_if()

Paul E. McKenney
2021-06-04T20:56:02Z

On Fri, Jun 04, 2021 at 12:18:43PM -0700, Linus Torvalds wrote:
> On Fri, Jun 4, 2021 at 12:09 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Again, semantics do matter, and I don't see how the compiler could
> > actually break the fundamental issue of "load->conditional->store is a
> > fundamental ordering even without memory barriers because of basic
> > causality", because you can't just arbitrarily generate speculative
> > stores that would be visible to others.
>
> This, after all, is why we trust that the *hardware* can't do it.
>
> Even if the hardware mis-speculates and goes down the wrong branch,
> and speculatively does the store when it shouldn't have, we don't
> care: we know that such a speculative store can not possibly become
> semantically visible (*) to other threads.
>
> For all the same reasons, I don't see how a compiler can violate
> causal ordering of the code (assuming, again, that the test is
> _meaningful_ - if we write nonsensical code, that's a different
> issue).

I am probably missing your point, but something like this:

if (READ_ONCE(x))
y = 42;
else
y = 1729;

Can in theory be transformed into something like this:

y = 1729;
if (READ_ONCE(x))
y = 42;

The usual way to prevent it is to use WRITE_ONCE().

Fortunately, register sets are large, and gcc manages to do a single
store and use only %eax.

Thanx, Paul

> If we have compilers that create speculative stores that are visible
> to other threads, we need to fix them.
>
> Linus
>
> (*) By "semantically visible" I intend to avoid the whole timing/cache
> pattern kind of non-semantic visibility that is all about the spectre
> leakage kind of things.