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    SubjectRe: [RFC] LKMM: Add volatile_if()
    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.

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