bugfix #144: Do not free kstack when the main thread exits

os/src/task/mod.rs

@@ -127,6 +127,13 @@ pub fn exit_current_and_run_next(exit_code: i32) {
  process_inner.memory_set.recycle_data_pages();
  // drop file descriptors
  process_inner.fd_table.clear();
+ // Remove all tasks except for the main thread itself.
+ // This is because we are still using the kstack under the TCB
+ // of the main thread. This TCB, including its kstack, will be
+ // deallocated when the process is reaped via waitpid.
+ while process_inner.tasks.len() > 1 {
+ process_inner.tasks.pop();
+ }
  }
  drop(process);
  // we do not have to save task context