From 5aa98ea3dc2b34d1d8d9fc8a0dc38c6da225dfa6 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (VMware)" Date: Wed, 5 Feb 2020 02:17:57 -0500 Subject: [PATCH] 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) Signed-off-by: Steven Rostedt (VMware) Signed-off-by: Sasha Levin --- kernel/trace/trace.h | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h index 757bb1bffed9..99af95e294d8 100644 --- a/kernel/trace/trace.h +++ b/kernel/trace/trace.h @@ -879,6 +879,11 @@ 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)) { @@ -926,6 +931,11 @@ 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());