On Wed, Jul 11, 2018 at 08:15:59AM -0700, Paul E. McKenney wrote:
On Wed, Jul 11, 2018 at 10:46:18AM -0400, Steven Rostedt wrote:
On Wed, 11 Jul 2018 07:27:44 -0700 "Paul E. McKenney" paulmck@linux.vnet.ibm.com wrote:
On Wed, Jul 11, 2018 at 09:00:03AM -0400, Steven Rostedt wrote:
On Wed, 11 Jul 2018 14:49:54 +0200 Peter Zijlstra peterz@infradead.org wrote:
On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:
it_func_ptr = rcu_dereference_sched((tp)->funcs); \I would convert to rcu_dereference_raw() to appease sparse. The fancy stuff below is pointless if you then turn off all checking.
The problem with doing this is if we use a trace event without the proper _idle() or whatever, we wont get a warning that it is used incorrectly with lockdep. Or does lockdep still check if "rcu is watching" with rcu_dereference_raw()?
No lockdep checking is done by rcu_dereference_raw().
Correct, but I think we can do this regardless. So Joel please resend with Peter's suggestion.
The reason being is because of this:
#define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args) \ extern struct tracepoint __tracepoint_##name; \ static inline void trace_##name(proto) \ { \ if (static_key_false(&__tracepoint_##name.key)) \ __DO_TRACE(&__tracepoint_##name, \ TP_PROTO(data_proto), \ TP_ARGS(data_args), \ TP_CONDITION(cond), 0); \ if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) { \ rcu_read_lock_sched_notrace(); \ rcu_dereference_sched(__tracepoint_##name.funcs);\ rcu_read_unlock_sched_notrace(); \ } \ }
Because lockdep would only trigger warnings when the tracepoint was enabled and used in a place it shouldn't be, we added the above IS_ENABLED(CONFIG_LOCKDEP) part to test regardless if the the tracepoint was enabled or not. Because we do this, we don't need to have the test in the __DO_TRACE() code itself. That means we can clean up the code as per Peter's suggestion.
Indeed, the rcu_dereference_sched() would catch it in that case, so agreed, Peter's suggestion isn't losing any debuggability.
Hmm, but if we are doing the check later anyway, then why not do it in __DO_TRACE itself?
Also I guess we are discussing about changing the rcu_dereference_sched which I think should go into a separate patch since my patch isn't touching how the rcuidle==0 paths use the RCU API. So I think this is an existing issue independent of this series.
thanks!
- Joel
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