Re: [PATCH v6 tip 2/8] tracing: attach BPF programs to kprobes

Peter Zijlstra
2015-03-12T15:15:29Z

On Tue, Mar 10, 2015 at 09:18:48PM -0700, Alexei Starovoitov wrote:
> +unsigned int trace_call_bpf(struct bpf_prog *prog, void *ctx)
> +{
> + unsigned int ret;
> + int cpu;
> +
> + if (in_nmi()) /* not supported yet */
> + return 1;
> +
> + preempt_disable_notrace();
> +
> + cpu = raw_smp_processor_id();
> + if (unlikely(per_cpu(bpf_prog_active, cpu)++ != 0)) {
> + /* since some bpf program is already running on this cpu,
> + * don't call into another bpf program (same or different)
> + * and don't send kprobe event into ring-buffer,
> + * so return zero here
> + */
> + ret = 0;
> + goto out;
> + }
> +
> + rcu_read_lock();

You've so far tried very hard to not get into tracing; and then you call
rcu_read_lock() :-)

So either document why this isn't a problem, provide
rcu_read_lock_notrace() or switch to RCU-sched and thereby avoid the
problem.

> + ret = BPF_PROG_RUN(prog, ctx);
> + rcu_read_unlock();
> +
> + out:
> + per_cpu(bpf_prog_active, cpu)--;
> + preempt_enable_notrace();
> +
> + return ret;
> +}
> +EXPORT_SYMBOL_GPL(trace_call_bpf);