Re: [PATCH 00/13] [RFC] Rust support

[Peter Zijlstra]

On Mon, Apr 19, 2021 at 09:53:06AM +0200, Paolo Bonzini wrote:
> On 19/04/21 09:32, Peter Zijlstra wrote:
> > On Sat, Apr 17, 2021 at 04:51:58PM +0200, Paolo Bonzini wrote:
> > > On 16/04/21 09:09, Peter Zijlstra wrote:
> > > > Well, the obvious example would be seqlocks. C11 can't do them
> > > 
> > > Sure it can.  C11 requires annotating with (the equivalent of) READ_ONCE all
> > > reads of seqlock-protected fields, but the memory model supports seqlocks
> > > just fine.
> > 
> > How does that help?
> > 
> > IIRC there's two problems, one on each side the lock. On the write side
> > we have:
> > 
> > 	seq++;
> > 	smp_wmb();
> > 	X = r;
> > 	Y = r;
> > 	smp_wmb();
> > 	seq++;
> > 
> > Which C11 simply cannot do right because it does't have wmb.
> 
> It has atomic_thread_fence(memory_order_release), and
> atomic_thread_fence(memory_order_acquire) on the read side.

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

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.