[PATCH 2/2] arm64: bpf: add BPF XADD instruction

[Alexei Starovoitov]

On Wed, Nov 11, 2015 at 04:23:41PM +0000, Will Deacon wrote:
> 
> If we're going to document it, a bug tracker might be a good place to
> start. The behaviour, as it stands, is broken wrt the definition of the
> __sync primitives. That is, there is no way to build __sync_fetch_and_add
> out of BPF_XADD without changing its semantics.

BPF_XADD == atomic_add() in kernel. period.
we are not going to deprecate it or introduce something else.
Semantics of __sync* or atomic in C standard and/or gcc/llvm has
nothing to do with this.
arm64 JIT needs to JIT bpf_xadd insn equivalent to the code
of atomic_add() which is 'stadd' in armv8.1.
The cpu check can be done by jit and for older cpus just fall back
to interpreter. trivial.

> We could fix this by either:
> 
> (1) Defining BPF_XADD to match __sync_fetch_and_add (including memory
>     barriers).

nope.

> (2) Introducing some new BPF_ atomics, that map to something like the
>     C11 __atomic builtins and deprecating BPF_XADD in favour of these.

nope.

> (3) Introducing new source-language intrinsics to match what BPF can do
>     (unlikely to be popular).

llvm's __sync intrinsic is used temporarily until we have time to do
new intrinsic in llvm that matches kernel's atomic_add() properly.
It will be done similar to llvm-bpf load_byte/word intrinsics.
Note that we've been hiding it under lock_xadd() wrapper, like here:
https://github.com/iovisor/bcc/blob/master/examples/networking/tunnel_monitor/monitor.c#L130