linux-kernel - Re: [PATCH 00/13] [RFC] Rust support

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Message-ID: <YH1PGfC1qSjKB6Ho@hirez.programming.kicks-ass.net>
Date:   Mon, 19 Apr 2021 11:36:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wedson Almeida Filho <wedsonaf@...gle.com>, ojeda@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 00/13] [RFC] Rust support

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.

I also don't see how this is better than seq_cst.

But yes, not broken, but also very much not optimal.