[3.8.y.z extended stable] Patch "ftrace: Fix synchronization location disabling and freeing ftrace_ops" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    ftrace: Fix synchronization location disabling and freeing ftrace_ops

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.18.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 88967350b3a64be77a0cc8a9c2dfa4e7cdce1712 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Red Hat)" <rostedt at goodmis.org>
Date: Mon, 13 Jan 2014 12:56:21 -0500
Subject: ftrace: Fix synchronization location disabling and freeing ftrace_ops

commit a4c35ed241129dd142be4cadb1e5a474a56d5464 upstream.

The synchronization needed after ftrace_ops are unregistered must happen
after the callback is disabled from becing called by functions.

The current location happens after the function is being removed from the
internal lists, but not after the function callbacks were disabled, leaving
the functions susceptible of being called after their callbacks are freed.

This affects perf and any externel users of function tracing (LTTng and
SystemTap).

Fixes: cdbe61bfe704 "ftrace: Allow dynamically allocated function tracers"
Signed-off-by: Steven Rostedt <rostedt at goodmis.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 kernel/trace/ftrace.c | 58 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 32 insertions(+), 26 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 0ae6f97..384cb73 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -396,20 +396,6 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops)
 	} else if (ops->flags & FTRACE_OPS_FL_CONTROL) {
 		ret = remove_ftrace_list_ops(&ftrace_control_list,
 					     &control_ops, ops);
-		if (!ret) {
-			/*
-			 * The ftrace_ops is now removed from the list,
-			 * 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()).
-			 */
-			schedule_on_each_cpu(ftrace_sync);
-			control_ops_free(ops);
-		}
 	} else
 		ret = remove_ftrace_ops(&ftrace_ops_list, ops);

@@ -419,17 +405,6 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops)
 	if (ftrace_enabled)
 		update_ftrace_function();

-	/*
-	 * 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)
-		schedule_on_each_cpu(ftrace_sync);
-
-
 	return 0;
 }

@@ -2096,10 +2071,41 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
 		command |= FTRACE_UPDATE_TRACE_FUNC;
 	}

-	if (!command || !ftrace_enabled)
+	if (!command || !ftrace_enabled) {
+		/*
+		 * If these are control ops, they still need their
+		 * per_cpu field freed. Since, function tracing is
+		 * not currently active, we can just free them
+		 * without synchronizing all CPUs.
+		 */
+		if (ops->flags & FTRACE_OPS_FL_CONTROL)
+			control_ops_free(ops);
 		return 0;
+	}

 	ftrace_run_update_code(command);
+
+	/*
+	 * Dynamic ops may be freed, we must make sure that all
+	 * callers are done before leaving this function.
+	 * The same goes for freeing the per_cpu data of the control
+	 * ops.
+	 *
+	 * Again, normal synchronize_sched() is not good enough.
+	 * We need to do a hard force of sched synchronization.
+	 * This is because we use preempt_disable() to do RCU, but
+	 * the function tracers can be called where RCU is not watching
+	 * (like before user_exit()). We can not rely on the RCU
+	 * infrastructure to do the synchronization, thus we must do it
+	 * ourselves.
+	 */
+	if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_CONTROL)) {
+		schedule_on_each_cpu(ftrace_sync);
+
+		if (ops->flags & FTRACE_OPS_FL_CONTROL)
+			control_ops_free(ops);
+	}
+
 	return 0;
 }

--
1.8.3.2