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Date:   Mon, 19 Apr 2021 12:01:21 +0100
From:   Will Deacon <will@kernel.org>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Wedson Almeida Filho <wedsonaf@google.com>, ojeda@kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/13] [RFC] Rust support
Message-ID: <20210419110120.GA29869@willie-the-truck>
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On Mon, Apr 19, 2021 at 11:40:54AM +0200, Paolo Bonzini wrote:
> On 19/04/21 11:36, Peter Zijlstra wrote:
> > On Mon, Apr 19, 2021 at 11:02:12AM +0200, Paolo Bonzini wrote:
> > > > void writer(void)
> > > > {
> > > >       atomic_store_explicit(&seq, seq+1, memory_order_relaxed);
> > > >       atomic_thread_fence(memory_order_acquire);
> > > 
> > > This needs to be memory_order_release.  The only change in the resulting
> > > assembly is that "dmb ishld" becomes "dmb ish", which is not as good as the
> > > "dmb ishst" you get from smp_wmb() but not buggy either.
> > 
> > Yuck! And that is what requires the insides to be
> > atomic_store_explicit(), otherwise this fence doesn't have to affect
> > them.
> 
> Not just that, even the write needs to be atomic_store_explicit in order to
> avoid a data race.atomic_store_explicit

https://wg21.link/P0690

was an attempt to address this, but I don't know if any of the ideas got
adopted in the end.

Will