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

[Paolo Bonzini]

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.

> Simlar thing for RCU; C11 can't optimally do that

Technically if you know what you're doing (i.e. that you're not on 
Alpha) you can do RCU using a relaxed load followed by an 
atomic_signal_fence(memory_order_consume).  Which I agree is horrible 
and not entirely within the standard, but it works in practice.  The 
Linux implementation of memory barriers, atomic RMW primitives, 
load-acquire/store-release etc. is also completely outside the standard, 
so it's not much different and more portable.

The only thing that I really, really miss when programming with C11 
atomics is smp_mb__{before,after}_atomic().

Paolo