tracing/timerlat: Add interface_lock around clearing of kthread in stop_kthread()

The timerlat interface will get and put the task that is part of the
"kthread" field of the osn_var to keep it around until all references are
released. But here's a race in the "stop_kthread()" code that will call
put_task_struct() on the kthread if it is not a kernel thread. This can
race with the releasing of the references to that task struct and the
put_task_struct() can be called twice when it should have been called just
once.

Take the interface_lock() in stop_kthread() to synchronize this change.
But to do so, the function stop_per_cpu_kthreads() needs to change the
loop from for_each_online_cpu() to for_each_possible_cpu() and remove the
cpu_read_lock(), as the interface_lock can not be taken while the cpu
locks are held. The only side effect of this change is that it may do some
extra work, as the per_cpu variables of the offline CPUs would not be set
anyway, and would simply be skipped in the loop.

Remove unneeded "return;" in stop_kthread().

Cc: [email protected]
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Tomas Glozar <[email protected]>
Cc: John Kacur <[email protected]>
Cc: "Luis Claudio R. Goncalves" <[email protected]>
Link: https://lore.kernel.org/[email protected]
Fixes: e88ed22 ("tracing/timerlat: Add user-space interface")
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

kernel/trace/trace_osnoise.c

@@ -1953,8 +1953,12 @@ static void stop_kthread(unsigned int cpu)
 {
 	struct task_struct *kthread;
 
+	mutex_lock(&interface_lock);
 	kthread = per_cpu(per_cpu_osnoise_var, cpu).kthread;
 	if (kthread) {
+		per_cpu(per_cpu_osnoise_var, cpu).kthread = NULL;
+		mutex_unlock(&interface_lock);
+
 		if (cpumask_test_and_clear_cpu(cpu, &kthread_cpumask) &&
 		    !WARN_ON(!test_bit(OSN_WORKLOAD, &osnoise_options))) {
 			kthread_stop(kthread);
@@ -1967,16 +1971,15 @@ static void stop_kthread(unsigned int cpu)
 			kill_pid(kthread->thread_pid, SIGKILL, 1);
 			put_task_struct(kthread);
 		}
-		per_cpu(per_cpu_osnoise_var, cpu).kthread = NULL;
 	} else {
+		mutex_unlock(&interface_lock);
 		/* if no workload, just return */
 		if (!test_bit(OSN_WORKLOAD, &osnoise_options)) {
 			/*
 			 * This is set in the osnoise tracer case.
 			 */
 			per_cpu(per_cpu_osnoise_var, cpu).sampling = false;
 			barrier();
-			return;
 		}
 	}
 }
@@ -1991,12 +1994,8 @@ static void stop_per_cpu_kthreads(void)
 {
 	int cpu;
 
-	cpus_read_lock();
-
-	for_each_online_cpu(cpu)
+	for_each_possible_cpu(cpu)
 		stop_kthread(cpu);
-
-	cpus_read_unlock();
 }
 
 /*