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

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Date:   Mon, 7 Jun 2021 21:07:58 +0300 (MSK)
From:   Alexander Monakov <amonakov@...ras.ru>
To:     Segher Boessenkool <segher@...nel.crashing.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...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 Mon, 7 Jun 2021, Segher Boessenkool wrote:

> > So the barrier which is a compiler barrier but not a machine barrier is
> > __atomic_signal_fence(model), but internally GCC will not treat it smarter
> > than an asm-with-memory-clobber today.
> 
> It will do nothing for relaxed ordering, and do blockage for everything
> else.  Can it do anything weaker than that?

It's a "blockage instruction" after transitioning to RTL, but before that,
on GIMPLE, the compiler sees it properly as a corresponding built-in, and
may optimize according to given memory model. And on RTL, well, if anyone
cares they'll need to invent RTL representation for it, I guess.

Alexander