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

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/04/21 10:26, Peter Zijlstra wrote:
> 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);

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.

The read side can use "dmb ishld" so it gets the same code as Linux.

LWN needs a "C11 memory model for kernel folks" article.  In the 
meanwhile there is 
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0124r4.html 
which is the opposite (Linux kernel memory model for C11 folks).

Paolo

> 
>      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.
> 
>