Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub

[Brendan Jackman]

On Tue, Dec 01, 2020 at 09:55:22PM -0800, Alexei Starovoitov wrote:
> On Tue, Dec 1, 2020 at 4:38 AM Brendan Jackman <jackmanb@xxxxxxxxxx> wrote:
> >
> > I guess it's also worth remembering other archs might have an atomic
> > subtract.
> 
> which one?
> arm64 LSE implements atomic_fetch_sub as neg+ldadd.
> imo x64 and arm64 example outweighs choices by other archs if there are such.
> Even without LSE it will be neg+llsc loop.
> The reason I proposed bpf xsub insn earlier is that I thought that llvm
> won't be able to emit it so easily and JIT/verifier would struggle.

Ack, I'll drop the atomic subtract instruction.