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authorSteven Rostedt (VMware) <rostedt@goodmis.org>2020-02-05 02:17:57 -0500
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2020-02-05 17:15:57 -0500
commit16052dd5bdfa16dbe18d8c1d4cde2ddab9d23177 (patch)
tree1e1a0196c967b872befb00d64b839492bf77c54b /kernel/trace
parentfd0e6852c407dd9aefc594f54ddcc21d84803d3b (diff)
ftrace: Add comment to why rcu_dereference_sched() is open coded
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@joelfernandes.org> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Diffstat (limited to 'kernel/trace')
-rw-r--r--kernel/trace/trace.h10
1 files changed, 10 insertions, 0 deletions
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());