Messages in this thread | | | Date | Tue, 4 Jun 2013 13:03:31 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize_sched() |
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On Tue, May 28, 2013 at 08:01:16PM -0400, Steven Rostedt wrote: > The function tracer uses preempt_disable/enable_notrace() for > synchronization between reading registered ftrace_ops and unregistering > them. > > Most of the ftrace_ops are global permanent structures that do not > require this synchronization. That is, ops may be added and removed from > the hlist but are never freed, and wont hurt if a synchronization is > missed. > > But this is not true for dynamically created ftrace_ops or control_ops, > which are used by the perf function tracing. > > The problem here is that the function tracer can be used to trace > kernel/user context switches as well as going to and from idle. > Basically, it can be used to trace blind spots of the RCU subsystem. > This means that even though preempt_disable() is done, a > synchronize_sched() will ignore CPUs that haven't made it out of user > space or idle. These can include functions that are being traced just > before entering or exiting the kernel sections. > > To implement the RCU synchronization, instead of using > synchronize_sched() the use of schedule_on_each_cpu() is performed. This > means that when a dynamically allocated ftrace_ops, or a control ops is > being unregistered, all CPUs must be touched and execute a ftrace_sync() > stub function via the work queues. This will rip CPUs out from idle or > in dynamic tick mode. This only happens when a user disables perf > function tracing or other dynamically allocated function tracers, but it > allows us to continue to debug RCU and context tracking with function > tracing. > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
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?
Thanks.
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