linux-kernel - Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy synchronize

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Date:	Tue, 4 Jun 2013 13:03:31 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Paul E. McKenney" <paulmck@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH] ftrace: Use schedule_on_each_cpu() as a heavy
 synchronize_sched()

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@...dmis.org>

Acked-by: Frederic Weisbecker <fweisbec@...il.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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