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    SubjectRe: [PATCH v6 tip 2/8] tracing: attach BPF programs to kprobes
    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);


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