Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 4, 2021 at 12:09 PM Linus Torvalds
[off-list ref] 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).

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.