From 1cd057a400520e8787d0d62ba32ccc0201a0d0b6 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 497defe9ed26..b769638f005c 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());