linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <CAHk-=wik7T+FoDAfqFPuMGVp6HxKYOf8UeKt3+EmovfivSgQ2Q@mail.gmail.com>
Date:   Fri, 4 Jun 2021 12:18:43 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 4, 2021 at 12:09 PM Linus Torvalds
<torvalds@...ux-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).

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.