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

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Date:   Mon, 19 Apr 2021 11:40:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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

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

It is better than seq_cst on TSO architectures.  Another possibility is 
to use release stores for everything (both increments and the stores 
between them).

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

Agreed on that, just like RCU/memory_order_consume.

Paolo