Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 19 Apr 2021 10:35:21 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     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,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH 00/13] [RFC] Rust support
Message-ID: <YH1Ayc6UncJ32uQZ@hirez.programming.kicks-ass.net>
References: <20210414184604.23473-1-ojeda@kernel.org>
 <YHiMyE4E1ViDcVPi@hirez.programming.kicks-ass.net>
 <YHj02M3jMSweoP4l@google.com>
 <YHk4DZE1ZWTiBB1f@hirez.programming.kicks-ass.net>
 <aa6e44ab-e223-73aa-279e-8103732460ac@redhat.com>
 <YH0yCTgL0raKrmYg@hirez.programming.kicks-ass.net>
 <7287eac3-f492-bab1-9ea8-b89ceceed560@redhat.com>
 <YH0+0VQ1XC8+rv20@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YH0+0VQ1XC8+rv20@hirez.programming.kicks-ass.net>
Precedence: bulk

On Mon, Apr 19, 2021 at 10:26:57AM +0200, Peter Zijlstra wrote:

> https://godbolt.org/z/85xoPxeE5

That wants _Atomic on the seq definition for clang.

> void writer(void)
> {
>     atomic_store_explicit(&seq, seq+1, memory_order_relaxed);
>     atomic_thread_fence(memory_order_acquire);
> 
>     X = 1;
>     Y = 2;
> 
>     atomic_store_explicit(&seq, seq+1, memory_order_release);
> }
> 
> gives:
> 
> writer:
>         adrp    x1, .LANCHOR0
>         add     x0, x1, :lo12:.LANCHOR0
>         ldr     w2, [x1, #:lo12:.LANCHOR0]
>         add     w2, w2, 1
>         str     w2, [x0]
>         dmb     ishld
>         ldr     w1, [x1, #:lo12:.LANCHOR0]
>         mov     w3, 1
>         mov     w2, 2
>         stp     w3, w2, [x0, 4]
>         add     w1, w1, w3
>         stlr    w1, [x0]
>         ret
> 
> Which, afaict, is completely buggered. What it seems to be doing is
> turning the seq load into a load-acquire, but what we really need is to
> make sure the seq store (increment) is ordered before the other stores.

Put differently, what you seem to want is store-acquire, but there ain't
no such thing.