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Date:   Mon, 7 Jun 2021 00:19:23 +0300 (MSK)
From:   Alexander Monakov <amonakov@ispras.ru>
To:     Linus Torvalds <torvalds@linux-foundation.org>
cc:     Jakub Jelinek <jakub@redhat.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>,
        Andrea Parri <parri.andrea@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nick Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        Akira Yokosawa <akiyks@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-toolchains@vger.kernel.org,
        linux-arch <linux-arch@vger.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()
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On Sun, 6 Jun 2021, Linus Torvalds wrote:

> On Sun, Jun 6, 2021 at 11:59 AM Jakub Jelinek <jakub@redhat.com> wrote:
> >
> > I think just
> > #define barrier() __asm__ __volatile__("" : : "i" (__COUNTER__) : "memory")
> > should be enough
> 
> Oh, I like that. Much better.
> 
> It avoids all the issues with comments etc, and because it's not using
> __COUNTER__ as a string, it doesn't need the preprocessor games with
> double expansion either.
> 
> So yeah, that seems like a nice solution to the issue, and should make
> the barriers all unique to the compiler.

It also plants a nice LTO time-bomb (__COUNTER__ values will be unique
only within each LTO input unit, not across all of them).

Alexander