On Tue, Jun 04, 2013 at 08:11:21AM -0400, Steven Rostedt wrote: > On Tue, 2013-06-04 at 13:03 +0200, Frederic Weisbecker wrote: > > > If ftrace were to use rcu_dereference_sched() instead of > > rcu_dereference_raw(), I guess > > the issue would have been detected before. But I guess we want to avoid > > that for > > tracing recursion? > > It's been detected before, just ignored as most of ftrace function > tracing has permanent objects and the synchronization doesn't really > matter for them. But for perf and SystemTap that uses dynamically > created ftrace_ops and needs to free them, it does make a difference. > Something I knew needed to be fixed but never got around to it as the > race is extremely tight (and requires root user trying to trigger it). I > haven't been ably to trigger the race, but in theory it's there. > > Note the checks that rcu_dereference_sched() has to test for these kinds > of things would cause ftrace to live lock the system if it had used it. > In fact, I had to make a rcu_dereference_raw_notrace() to prevent ftrace > locking up the system when full rcu debugging is enabled.
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