add comment to start discussion

tokio/src/runtime/blocking/pool.rs

@@ -270,6 +270,10 @@ impl Inner {
             shared.num_idle += 1;
 
             while !shared.shutdown {
+                // NOTE: A task spawned by spawn_blocking if:
+                // 1. It is spawned (thus the `condvar` gets a `notify_one`)
+                // 2. `shutdown` is called and manages to acquire the `shared.lock` *before* the
+                //    thread woken up here gets a hold of it. Then `shutdown` will be true.
                 let lock_result = self.condvar.wait_timeout(shared, self.keep_alive).unwrap();
 
                 shared = lock_result.0;