Re: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 20, 2025 at 01:01:18AM +0800, Tao Chen wrote:
> As Alexei noted, get_perf_callchain() return values may be reused
> if a task is preempted after the BPF program enters migrate disable
> mode. Drawing on the per-cpu design of bpf_bprintf_buffers,
> per-cpu BPF callchain entry is used here.

And now you can only unwind 3 tasks, and then start failing. This is
acceptable, why?

> -	if (may_fault)
> -		rcu_read_lock(); /* need RCU for perf's callchain below */
> -

I know you propose to remove this code; but how was that correct? The
perf callchain code hard relies on non-preemptible context, RCU does not
imply such a thing.

>  	if (trace_in)
>  		trace = trace_in;
> -	else if (kernel && task)
>  		trace = get_callchain_entry_for_task(task, max_depth);
> -	else
> -		trace = get_perf_callchain(regs, NULL, kernel, user, max_depth,
> -					   crosstask, false);