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.
You end up
having to use seq_cst for the first wmb or make both X and Y (on top of
the last seq) a store-release, both options are sub-optimal.
seq_cst (except for the fence which is just smp_mb) is a pile of manure,
no doubt about that. :)
Paolo
