3.8.13.18 -stable review patch. If anyone has any objections, please let me know. ------------------ From: Steven Rostedt <rostedt@xxxxxxxxxxx> commit 7614c3dc74733dff4b0e774f7a894b9ea6ec508c upstream. 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. Link: http://lkml.kernel.org/r/1369785676.15552.55.camel@xxxxxxxxxxxxxxxxxx Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Tejun Heo <tj@xxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx> [ kamal: 3.8-stable prereq for a4c35ed2 "ftrace: Fix synchronization location disabling and freeing ftrace_ops" ] Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx> --- kernel/trace/ftrace.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index c7f6959..0ae6f97 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c @@ -367,6 +367,17 @@ static int __register_ftrace_function(struct ftrace_ops *ops) return 0; } +static void ftrace_sync(struct work_struct *work) +{ + /* + * This function is just a stub to implement a hard force + * of synchronize_sched(). This requires synchronizing + * tasks even in userspace and idle. + * + * Yes, function tracing is rude. + */ +} + static int __unregister_ftrace_function(struct ftrace_ops *ops) { int ret; @@ -391,8 +402,12 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops) * so there'll be no new users. We must ensure * all current users are done before we free * the control data. + * Note synchronize_sched() is not enough, as we + * use preempt_disable() to do RCU, but the function + * tracer can be called where RCU is not active + * (before user_exit()). */ - synchronize_sched(); + schedule_on_each_cpu(ftrace_sync); control_ops_free(ops); } } else @@ -407,9 +422,13 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops) /* * Dynamic ops may be freed, we must make sure that all * callers are done before leaving this function. + * + * Again, normal synchronize_sched() is not good enough. + * We need to do a hard force of sched synchronization. */ if (ops->flags & FTRACE_OPS_FL_DYNAMIC) - synchronize_sched(); + schedule_on_each_cpu(ftrace_sync); + return 0; } -- 1.8.3.2 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html