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. 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.
On the read side you get:
do {
s = seq;
smp_rmb();
r1 = X;
r2 = Y;
smp_rmb();
} while ((s&1) || seq != s);
And then you get into trouble the last barrier, so the first seq load
can be load-acquire, after which the loads of X, Y come after, but you
need then to happen before the second seq load, for which you then need
seq_cst, or make X and Y load-acquire. Again, not optimal.
I have also seen *many* broken variants of it on the web. Some work on
x86 but are totally broken when you build them on LL/SC ARM64.
