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

[Peter Zijlstra]

On Wed, Nov 11, 2015 at 03:40:15PM -0800, Alexei Starovoitov wrote:
> On Wed, Nov 11, 2015 at 11:21:35PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 11, 2015 at 11:55:59AM -0800, Alexei Starovoitov wrote:
> > > Therefore things like memory barriers, full set of atomics are not applicable
> > > in bpf world.
> > 
> > There are still plenty of wait-free constructs one can make using them.
> 
> yes, but all such lock-free algos are typically based on cmpxchg8b and
> tight loop, so it would be very hard for verifier to proof termination
> of such loops. I think when we'd need to add something like this, we'll
> add new bpf insn that will be membarrier+cmpxhg8b+check+loop as
> a single insn, so it cannot be misused.
> I don't know of any concrete use case yet. All possible though.

So this is where the 'unconditional' atomic ops come in handy.

Like the x86: xchg, lock {xadd,add,sub,inc,dec,or,and,xor}

Those do not have a loop, and then you can create truly wait-free
things; even some applications of cmpxchg do not actually need the loop.

But this class of wait-free constructs is indeed significantly smaller
than the class of lock-less constructs.

> btw, support for mini loops was requested many times in the past.
> I guess we'd have to add something like this, but it's tricky.
> Mainly because control flow graph analysis becomes much more complicated.

Agreed, that does sound like an 'interesting' problem :-)

Something like:

atomic_op(ptr, f)
{
	for (;;) {
		val = *ptr;
		new = f(val)
		old = cmpxchg(ptr, val, new);
		if (old == val)
			break;

		cpu_relax();
	}
}

might be castable as an instruction I suppose, but I'm not sure you have
function references in (e)BPF.

The above is 'sane' if f is sane (although there is a
starvation case, which is why things like sparc (iirc) need an
increasing backoff instead of cpu_relax()).