Re: [PATCH v4 0/3] sched_ext: Harden scx_bpf_cpu_rq()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 11, 2025 at 10:21:47PM +0100, Christian Loehle wrote:
> scx_bpf_cpu_rq() currently allows accessing struct rq fields without
> holding the associated rq.
> It is being used by scx_cosmos, scx_flash, scx_lavd, scx_layered, and
> scx_tickless. Fortunately it is only ever used to fetch rq->curr.
> So provide an alternative scx_bpf_task_acquire_remote_curr() that
> doesn't expose struct rq and provide a hardened scx_bpf_cpu_rq_locked()
> by ensuring we hold the rq lock.
> Add a deprecation warning to scx_bpf_cpu_rq_locked() that mentions the
> two alternatives.
> 
> This also simplifies scx code from:
> 
> rq = scx_bpf_cpu_rq(cpu);
> if (!rq)
> 	return;
> p = rq->curr
> if (!p)
> 	return;
> /* ... Do something with p */
> 
> into:
> 
> p = scx_bpf_task_acquire_remote_curr(cpu);
> if (!p)
> 	return;
> /* ... Do something with p */
> bpf_task_release(p);

Why do that mandatory refcount dance, rather than directly exposing the
RCU-ness of that pointer? IIRC BPF was good with RCU.