tools/deferred_work/3-4-5-deferred: Protect pid_task() with 'rcu_read…

…_lock()'

'get_proc()' in 'deferred.c' is calling 'pid_task()' without holding the
rcu read lock.  This result in below lockdep warning:

    =============================
    WARNING: suspicious RCU usage
    4.19.0+ #4 Tainted: G           O
    -----------------------------
    kernel/pid.c:330 suspicious rcu_dereference_check() usage!

This commit fixes it.

Signed-off-by: SeongJae Park <[email protected]>

tools/labs/templates/deferred_work/3-4-5-deferred/kernel/deferred.c

@@ -61,7 +61,9 @@ static struct mon_proc *get_proc(pid_t pid)
 	struct task_struct *task;
 	struct mon_proc *p;
 
+	rcu_read_lock();
 	task = pid_task(find_vpid(pid), PIDTYPE_PID);
+	rcu_read_unlock();
 	if (!task)
 		return ERR_PTR(-ESRCH);