rcu/ftrace: Add rcu_dereference_raw_notrace() and friends

As rcu_dereference_raw() under RCU debug config options can add quite a bit of checks, and that tracing uses rcu_dereference_raw(), these checks happen with the function tracer. The function tracer also happens to trace these debug checks too. This added overhead can livelock the system.

Add a new interface to RCU for both rcu_dereference_raw_notrace() as well as hlist_for_each_entry_rcu_notrace() as the hlist iterator uses the rcu_dereference_raw() as well, and is used a bit with the function tracer.

Signed-off-by Steven Rostedt <[email protected]>

Index: linux-trace.git/include/linux/rculist.h
===================================================================

include/linux/rculist.h +20 -0

@@ -461,6 +461,26 @@ static inline void hlist_add_after_rcu(s
 			&(pos)->member)), typeof(*(pos)), member))
 
 /**
+ * hlist_for_each_entry_rcu_notrace - iterate over rcu list of given type (for tracing)
+ * @pos:	the type * to use as a loop cursor.
+ * @head:	the head for your list.
+ * @member:	the name of the hlist_node within the struct.
+ *
+ * This list-traversal primitive may safely run concurrently with
+ * the _rcu list-mutation primitives such as hlist_add_head_rcu()
+ * as long as the traversal is guarded by rcu_read_lock().
+ *
+ * This is the same as hlist_for_each_entry_rcu() except that it does
+ * not do any RCU debugging or tracing.
+ */
+#define hlist_for_each_entry_rcu_notrace(pos, head, member)			\
+	for (pos = hlist_entry_safe (rcu_dereference_raw_notrace(hlist_first_rcu(head)),\
+			typeof(*(pos)), member);			\
+		pos;							\
+		pos = hlist_entry_safe(rcu_dereference_raw_notrace(hlist_next_rcu(\
+			&(pos)->member)), typeof(*(pos)), member))
+
+/**
  * hlist_for_each_entry_rcu_bh - iterate over rcu list of given type
  * @pos:	the type * to use as a loop cursor.
  * @head:	the head for your list.

Index: linux-trace.git/include/linux/rcupdate.h
===================================================================

include/linux/rcupdate.h +9 -0

@@ -640,6 +640,15 @@ static inline void rcu_preempt_sleep_che
 
 #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ needed? @@@*/
 
+/*
+ * The tracing infrastructure traces RCU (we want that), but unfortunately
+ * some of the RCU checks causes tracing to lock up the system.
+ *
+ * The tracing version of rcu_dereference_raw() must not call
+ * rcu_read_lock_held().
+ */
+#define rcu_dereference_raw_notrace(p) __rcu_dereference_check((p), 1, __rcu)
+
 /**
  * rcu_access_index() - fetch RCU index with no dereferencing
  * @p: The index to read

As rcu_dereference_raw() under RCU debug config options can add quite a bit of checks, and that tracing uses rcu_dereference_raw(), these checks happen with the function tracer. The function tracer also happens to trace these debug checks too. This added overhead can livelock the system.

Have the function tracer use the new RCU _notrace equivalents that do not do the debug checks for RCU.

Signed-off-by Steven Rostedt <[email protected]>

Index: linux-trace.git/kernel/trace/ftrace.c
===================================================================

kernel/trace/ftrace.c +9 -9

@@ -120,22 +120,22 @@ static void ftrace_ops_no_ops(unsigned l
 
 /*
  * Traverse the ftrace_global_list, invoking all entries.  The reason that we
- * can use rcu_dereference_raw() is that elements removed from this list
+ * can use rcu_dereference_raw_notrace() 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
+ * mechanism.  The rcu_dereference_raw_notrace() calls are needed to handle
  * concurrent insertions into the ftrace_global_list.
  *
  * Silly Alpha and silly pointer-speculation compiler optimizations!
  */
 #define do_for_each_ftrace_op(op, list)			\
-	op = rcu_dereference_raw(list);			\
+	op = rcu_dereference_raw_notrace(list);			\
 	do
 
 /*
  * Optimized for just a single item in the list (as that is the normal case).
  */
 #define while_for_each_ftrace_op(op)				\
-	while (likely(op = rcu_dereference_raw((op)->next)) &&	\
+	while (likely(op = rcu_dereference_raw_notrace((op)->next)) &&	\
 	       unlikely((op) != &ftrace_list_end))
 
 static inline void ftrace_ops_init(struct ftrace_ops *ops)

@@ -779,7 +779,7 @@ ftrace_find_profiled_func(struct ftrace_
 	if (hlist_empty(hhd))
 		return NULL;
 
-	hlist_for_each_entry_rcu(rec, hhd, node) {
+	hlist_for_each_entry_rcu_notrace(rec, hhd, node) {
 		if (rec->ip == ip)
 			return rec;
 	}

@@ -1165,7 +1165,7 @@ ftrace_lookup_ip(struct ftrace_hash *has
 
 	hhd = &hash->buckets[key];
 
-	hlist_for_each_entry_rcu(entry, hhd, hlist) {
+	hlist_for_each_entry_rcu_notrace(entry, hhd, hlist) {
 		if (entry->ip == ip)
 			return entry;
 	}

@@ -1422,8 +1422,8 @@ ftrace_ops_test(struct ftrace_ops *ops, 
 	struct ftrace_hash *notrace_hash;
 	int ret;
 
-	filter_hash = rcu_dereference_raw(ops->filter_hash);
-	notrace_hash = rcu_dereference_raw(ops->notrace_hash);
+	filter_hash = rcu_dereference_raw_notrace(ops->filter_hash);
+	notrace_hash = rcu_dereference_raw_notrace(ops->notrace_hash);
 
 	if ((ftrace_hash_empty(filter_hash) ||
 	     ftrace_lookup_ip(filter_hash, ip)) &&

@@ -2920,7 +2920,7 @@ static void function_trace_probe_call(un
 	 * on the hash. rcu_read_lock is too dangerous here.
 	 */
 	preempt_disable_notrace();
-	hlist_for_each_entry_rcu(entry, hhd, node) {
+	hlist_for_each_entry_rcu_notrace(entry, hhd, node) {
 		if (entry->ip == ip)
 			entry->ops->func(ip, parent_ip, &entry->data);
 	}