[PATCH 1/1] Fix: trace sched switch start/stop racy updates

[Mathieu Desnoyers]

Reading the sched_cmdline_ref and sched_tgid_ref initial state within
tracing_start_sched_switch without holding the sched_register_mutex is
racy against concurrent updates, which can lead to tracepoint probes
being registered more than once (and thus trigger warnings within
tracepoint.c).

Also, write and read to/from those variables should be done with
WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
probes without holding the sched_register_mutex.

[ Compile-tested only. I suspect it might fix the following syzbot
  report:

  syzbot+774fddf07b7ab29a1e55@xxxxxxxxxxxxxxxxxxxxxxxxx ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
CC: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
 kernel/trace/trace_sched_switch.c | 32 ++++++++++++++++++++++----------
 1 file changed, 22 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace_sched_switch.c b/kernel/trace/trace_sched_switch.c
index e288168661e1..902e8bf59aeb 100644
--- a/kernel/trace/trace_sched_switch.c
+++ b/kernel/trace/trace_sched_switch.c
@@ -26,8 +26,8 @@ probe_sched_switch(void *ignore, bool preempt,
 {
 	int flags;
 
-	flags = (RECORD_TGID * !!sched_tgid_ref) +
-		(RECORD_CMDLINE * !!sched_cmdline_ref);
+	flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
+		(RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));
 
 	if (!flags)
 		return;
@@ -39,8 +39,8 @@ probe_sched_wakeup(void *ignore, struct task_struct *wakee)
 {
 	int flags;
 
-	flags = (RECORD_TGID * !!sched_tgid_ref) +
-		(RECORD_CMDLINE * !!sched_cmdline_ref);
+	flags = (RECORD_TGID * !!READ_ONCE(sched_tgid_ref)) +
+		(RECORD_CMDLINE * !!READ_ONCE(sched_cmdline_ref));
 
 	if (!flags)
 		return;
@@ -89,21 +89,28 @@ static void tracing_sched_unregister(void)
 
 static void tracing_start_sched_switch(int ops)
 {
-	bool sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
+	bool sched_register;
+
 	mutex_lock(&sched_register_mutex);
+	sched_register = (!sched_cmdline_ref && !sched_tgid_ref);
 
 	switch (ops) {
 	case RECORD_CMDLINE:
-		sched_cmdline_ref++;
+		WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref + 1);
 		break;
 
 	case RECORD_TGID:
-		sched_tgid_ref++;
+		WRITE_ONCE(sched_tgid_ref, sched_tgid_ref + 1);
 		break;
+
+	default:
+		WARN_ONCE(1, "Unsupported tracing op: %d", ops);
+		goto end;
 	}
 
-	if (sched_register && (sched_cmdline_ref || sched_tgid_ref))
+	if (sched_register)
 		tracing_sched_register();
+end:
 	mutex_unlock(&sched_register_mutex);
 }
 
@@ -113,16 +120,21 @@ static void tracing_stop_sched_switch(int ops)
 
 	switch (ops) {
 	case RECORD_CMDLINE:
-		sched_cmdline_ref--;
+		WRITE_ONCE(sched_cmdline_ref, sched_cmdline_ref - 1);
 		break;
 
 	case RECORD_TGID:
-		sched_tgid_ref--;
+		WRITE_ONCE(sched_tgid_ref, sched_tgid_ref - 1);
 		break;
+
+	default:
+		WARN_ONCE(1, "Unsupported tracing op: %d", ops);
+		goto end;
 	}
 
 	if (!sched_cmdline_ref && !sched_tgid_ref)
 		tracing_sched_unregister();
+end:
 	mutex_unlock(&sched_register_mutex);
 }
 
-- 
2.11.0