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Subject[tip:core/urgent] ftrace: Replace read_barrier_depends() with rcu_dereference_raw()
Commit-ID:  3f379b03fbfddd20536389a85c6456f8233d1f8d
Gitweb: http://git.kernel.org/tip/3f379b03fbfddd20536389a85c6456f8233d1f8d
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
AuthorDate: Fri, 5 Mar 2010 15:03:25 -0800
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Thu, 11 Mar 2010 13:38:01 +0100

ftrace: Replace read_barrier_depends() with rcu_dereference_raw()

Replace the calls to read_barrier_depends() in
ftrace_list_func() with rcu_dereference_raw() to improve
readability. The reason that we use rcu_dereference_raw() here
is that removed entries are never freed, instead they are simply
leaked. This is one of a very few cases where use of
rcu_dereference_raw() is the long-term right answer. And I
don't yet know of any others. ;-)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1267830207-9474-1-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/trace/ftrace.c | 22 +++++++++++++---------
1 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 8378357..8c5adc0 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -27,6 +27,7 @@
#include <linux/ctype.h>
#include <linux/list.h>
#include <linux/hash.h>
+#include <linux/rcupdate.h>

#include <trace/events/sched.h>

@@ -88,18 +89,22 @@ ftrace_func_t ftrace_pid_function __read_mostly = ftrace_stub;
static int ftrace_set_func(unsigned long *array, int *idx, char *buffer);
#endif

+/*
+ * Traverse the ftrace_list, invoking all entries. The reason that we
+ * can use rcu_dereference_raw() is that elements removed from this list
+ * are simply leaked, so there is no need to interact with a grace-period
+ * mechanism. The rcu_dereference_raw() calls are needed to handle
+ * concurrent insertions into the ftrace_list.
+ *
+ * Silly Alpha and silly pointer-speculation compiler optimizations!
+ */
static void ftrace_list_func(unsigned long ip, unsigned long parent_ip)
{
- struct ftrace_ops *op = ftrace_list;
-
- /* in case someone actually ports this to alpha! */
- read_barrier_depends();
+ struct ftrace_ops *op = rcu_dereference_raw(ftrace_list); /*see above*/

while (op != &ftrace_list_end) {
- /* silly alpha */
- read_barrier_depends();
op->func(ip, parent_ip);
- op = op->next;
+ op = rcu_dereference_raw(op->next); /*see above*/
};
}

@@ -154,8 +159,7 @@ static int __register_ftrace_function(struct ftrace_ops *ops)
* the ops->next pointer is valid before another CPU sees
* the ops pointer included into the ftrace_list.
*/
- smp_wmb();
- ftrace_list = ops;
+ rcu_assign_pointer(ftrace_list, ops);

if (ftrace_enabled) {
ftrace_func_t func;

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