linux-kernel - [RFC][BUG] tracer: Fails to work

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Message-ID: <20160128080859.GB6357@twins.programming.kicks-ass.net>
Date:	Thu, 28 Jan 2016 09:08:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [RFC][BUG] tracer: Fails to work

Hi Steve,

So I was hunting wabbits the other day and ftrace failed to work.

After I cursed a bit on IRC, Thomas found the below, after 'fixing' it
like so things worked enough to get the trace out.

Relying on things like this make it entirely impossible to get any trace
data out if you've wedged a CPU. Exactly the kind of situation you want
trace data for.

Please consider an appropriate change to make this happen.

---
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 95181e36891a..b09c5b955555 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -4053,7 +4053,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_read_prepare);
 void
 ring_buffer_read_prepare_sync(void)
 {
-	synchronize_sched();
+//	synchronize_sched();
 }
 EXPORT_SYMBOL_GPL(ring_buffer_read_prepare_sync);