linux-kernel - [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference

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Message-Id: <20200205105113.283672584@goodmis.org>
Date:   Wed, 05 Feb 2020 05:49:33 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     linux-kernel@...r.kernel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [for-next][PATCH 4/4] ftrace: Add comment to why rcu_dereference_sched() is open coded

From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

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).

Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/trace/trace.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 022def96d307..8c52f5de9384 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -975,6 +975,11 @@ 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)) {
@@ -1022,6 +1027,11 @@ 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());
 
-- 
2.24.1