do_notify_parent_cldstop(current, false, why);
/*
- * Don't want to allow preemption here, because
- * sys_ptrace() needs this task to be inactive.
+ * The previous do_notify_parent_cldstop() invocation woke ptracer.
+ * One a PREEMPTION kernel this can result in preemption requirement
+ * which will be fulfilled after read_unlock() and the ptracer will be
+ * put on the CPU.
+ * The ptracer is in wait_task_inactive(, __TASK_TRACED) waiting for
+ * this task wait in schedule(). If this task gets preempted then it
+ * remains enqueued on the runqueue. The ptracer will observe this and
+ * then sleep for a delay of one HZ tick. In the meantime this task
+ * gets scheduled, enters schedule() and will wait for the ptracer.
*
- * XXX: implement read_unlock_no_resched().
+ * This preemption point is not bad from correctness point of view but
+ * extends the runtime by one HZ tick time due to the ptracer's sleep.
+ * The preempt-disable section ensures that there will be no preemption
+ * between unlock and schedule() and so improving the performance since
+ * the ptracer has no reason to sleep.
+ *
+ * On PREEMPT_RT locking tasklist_lock does not disable preemption.
+ * Therefore the task can be preempted (after
+ * do_notify_parent_cldstop()) before unlocking tasklist_lock so there
+ * is no benefit in doing this. The optimisation is harmful on
+ * PEEMPT_RT because the spinlock_t (in cgroup_enter_frozen()) must not
+ * be acquired with disabled preemption.
*/
- preempt_disable();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ preempt_disable();
read_unlock(&tasklist_lock);
cgroup_enter_frozen();
- preempt_enable_no_resched();
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ preempt_enable_no_resched();
schedule();
cgroup_leave_frozen(true);