On Mon, Oct 23, 2017 at 10:58:04AM -0700, Yonghong Song wrote:
> This patch enables multiple bpf attachments for a
> kprobe/uprobe/tracepoint single trace event.
This forgets to explain _why_ this is a good thing to do.
> +static DEFINE_MUTEX(bpf_event_mutex);
> +
> +int perf_event_attach_bpf_prog(struct perf_event *event,
> + struct bpf_prog *prog)
> +{
> + struct bpf_prog_array __rcu *old_array;
> + struct bpf_prog_array *new_array;
> + int ret;
> +
> + mutex_lock(&bpf_event_mutex);
> +
> + if (event->prog)
> + return -EEXIST;
> +
> + old_array = rcu_dereference_protected(event->tp_event->prog_array,
> +
> lockdep_is_held(&bpf_event_mutex));
Since all modifications to prog_array are serialized by this one mutex;
you don't need rcu_dereference() here, there are no possible ordering
problems.
> + ret = bpf_prog_array_copy(old_array, NULL, prog, &new_array);
> + if (ret < 0)
> + goto out;
> +
> + /* set the new array to event->tp_event and set event->prog */
> + event->prog = prog;
> + rcu_assign_pointer(event->tp_event->prog_array, new_array);
> +
> + if (old_array)
> + bpf_prog_array_free(old_array);
> +
> +out:
Its customary to call that unlock:
> + mutex_unlock(&bpf_event_mutex);
> + return ret;
> +}
