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
