On Sat, Feb 26, 2011 at 10:40:32AM +0800, zhao bao wrote:
Hello,everybody. When I read tracepoint code, I find variable __data never used. Am I missing something?
#define __DO_TRACE(tp, proto, args, cond) \ do { \ struct tracepoint_func *it_func_ptr; \ void *it_func; \ void *__data; \ \ if (!(cond)) \ return; \ rcu_read_lock_sched_notrace(); \ it_func_ptr = rcu_dereference_sched((tp)->funcs); \ if (it_func_ptr) { \ do { \ it_func = (it_func_ptr)->func; \ __data = (it_func_ptr)->data; \ ((void(*)(proto))(it_func))(args); \ } while ((++it_func_ptr)->func); \ } \ rcu_read_unlock_sched_notrace(); \ } while (0)
Yeah that's a quite tricky part. So what we want with tracepoints is to have them passing something specific as a first parameter. Always the same thing for a given func. So you register a probe with tracepoint_probe_register() and the third argument of this func, data, is the first parameter that will always been passed to your probe function. This is that "__data". We declare a tracepoint with DECLARE_TRACE(): DECLARE_TRACE(my_trace, int myarg, myarg). The CPP engine will translate that too: __DECLARE_TRACE(mytrace, (int myarg), (myarg), 1 (void *__data, int myarg), (__data, myarg)) ^^ Look, that where is the trick. DECLARE_TRACE is cheating by adding this __data argument. Further, __DO_TRACE will be called with these arguments: __DO_TRACE(&__tracepoint_mytrace, (void *__data, int myarg), (__data, myarg), (cond)) And then it makes the trick inside __DO_TRACE(), we end up having: void *__data; __data = (it_func_ptr)->data; ((void(*)(proto))(it_func))(__data, myarg); See? That's a kind of ghost argument we inject in our CPP macros and in the end we cheat in order to pass that constant tracepoint data as a first argument of the probe. In practice we use that to pass the ftrace event (struct ftrace_event_call *) structure as a first argument of the probe. It's a shame we needed to get something so unreadable and quick to generate headaches but IIRC we didn't have the choice in order to pass that specific data argument.