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 5.4-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-5.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 25ab8ebd2345274483e666c265c9baee751c6818 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 c4fd5731d6b37..08647723cfab9 100644 --- a/kernel/trace/trace.h +++ b/kernel/trace/trace.h @@ -943,6 +943,11 @@ 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)) { @@ -990,6 +995,11 @@ 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());