This is a note to let you know that I've just added the patch titled
ftrace: Add comment to why rcu_dereference_sched() is open coded
to the 5.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ftrace-add-comment-to-why-rcu_dereference_sched-is-o.patch
and it can be found in the queue-5.5 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6112c1ba351c997f1ba2a0f7ab0bc3091f78819d
Author: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Date: Wed Feb 5 02:17:57 2020 -0500
ftrace: Add comment to why rcu_dereference_sched() is open coded
[ Upstream commit 16052dd5bdfa16dbe18d8c1d4cde2ddab9d23177 ]
Because the function graph tracer can execute in sections where RCU is not
"watching", the rcu_dereference_sched() for the has needs to be open coded.
This is fine because the RCU "flavor" of the ftrace hash is protected by
its own RCU handling (it does its own little synchronization on every CPU
and does not rely on RCU sched).
Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 497defe9ed264..b769638f005c7 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -961,6 +961,11 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
preempt_disable_notrace();
+ /*
+ * Have to open code "rcu_dereference_sched()" because the
+ * function graph tracer can be called when RCU is not
+ * "watching".
+ */
hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());
if (ftrace_hash_empty(hash)) {
@@ -1008,6 +1013,11 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
preempt_disable_notrace();
+ /*
+ * Have to open code "rcu_dereference_sched()" because the
+ * function graph tracer can be called when RCU is not
+ * "watching".
+ */
notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
!preemptible());
