This is a note to let you know that I've just added the patch titled
ftrace: Protect ftrace_graph_hash with ftrace_sync
to the 5.5-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-protect-ftrace_graph_hash-with-ftrace_sync.patch
and it can be found in the queue-5.5 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit b9e00433bbd09bfa2baf704e02eb3e817d81b36b
Author: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Date: Wed Feb 5 09:20:32 2020 -0500
ftrace: Protect ftrace_graph_hash with ftrace_sync
[ Upstream commit 54a16ff6f2e50775145b210bcd94d62c3c2af117 ]
As function_graph tracer can run when RCU is not "watching", it can not be
protected by synchronize_rcu() it requires running a task on each CPU before
it can be freed. Calling schedule_on_each_cpu(ftrace_sync) needs to be used.
Link: https://lore.kernel.org/r/20200205131110.GT2935@paulmck-ThinkPad-P72
Cc: stable@xxxxxxxxxxxxxxx
Fixes: b9b0c831bed26 ("ftrace: Convert graph filter to use hash tables")
Reported-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Reviewed-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/ftrace.c b/kernel/trace/ftrace.c
index e85668cdd8c73..3581bd96d6eb3 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5872,8 +5872,15 @@ ftrace_graph_release(struct inode *inode, struct file *file)
mutex_unlock(&graph_lock);
- /* Wait till all users are no longer using the old hash */
- synchronize_rcu();
+ /*
+ * We need to do a hard force of sched synchronization.
+ * This is because we use preempt_disable() to do RCU, but
+ * the function tracers can be called where RCU is not watching
+ * (like before user_exit()). We can not rely on the RCU
+ * infrastructure to do the synchronization, thus we must do it
+ * ourselves.
+ */
+ schedule_on_each_cpu(ftrace_sync);
free_ftrace_hash(old_hash);
}
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index b769638f005c7..85f475bb48238 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -965,6 +965,7 @@ static inline int ftrace_graph_addr(struct ftrace_graph_ent *trace)
* Have to open code "rcu_dereference_sched()" because the
* function graph tracer can be called when RCU is not
* "watching".
+ * Protected with schedule_on_each_cpu(ftrace_sync)
*/
hash = rcu_dereference_protected(ftrace_graph_hash, !preemptible());
@@ -1017,6 +1018,7 @@ static inline int ftrace_graph_notrace_addr(unsigned long addr)
* Have to open code "rcu_dereference_sched()" because the
* function graph tracer can be called when RCU is not
* "watching".
+ * Protected with schedule_on_each_cpu(ftrace_sync)
*/
notrace_hash = rcu_dereference_protected(ftrace_graph_notrace_hash,
!preemptible());
