linux-kernel - Re: Testing tracer wakeup_rt: .. no entries found ..FAILED!

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Date:	Tue, 31 Jul 2012 20:09:38 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Fengguang Wu <fengguang.wu@...el.com>, Avi Kivity <avi@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: Testing tracer wakeup_rt: .. no entries found ..FAILED!

On Tue, 2012-07-31 at 16:57 -0700, Paul E. McKenney wrote:

> > What was the next lines? I bet you it was "PASSED". Which means it did
> > not fail. This is the second bug you found that has to do with RCU being
> > called in 'idle'. The one that Paul posted a patch for.
> 
> Though it needs another patch to actually use it in the right place...

Right. Something like this:

-- Steve

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 5638104..d915638 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -631,7 +631,12 @@ __update_max_tr(struct trace_array *tr, struct task_struct *tsk, int cpu)
 
 	memcpy(max_data->comm, tsk->comm, TASK_COMM_LEN);
 	max_data->pid = tsk->pid;
-	max_data->uid = task_uid(tsk);
+	/*
+	 * task_uid() calls rcu_read_lock, but this can be called
+	 * outside of RCU state monitoring (irq going back to idle).
+	 */ 
+	RCU_NONIDLE(max_data->uid = task_uid(tsk));
+
 	max_data->nice = tsk->static_prio - 20 - MAX_RT_PRIO;
 	max_data->policy = tsk->policy;
 	max_data->rt_priority = tsk->rt_priority;


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