[PATCH v2 4/6] rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks

[Boqun Feng]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

This commit continues the elimination of deadlocks involving do_exit()
and RCU tasks by causing exit_tasks_rcu_start() to add the current
task to a per-CPU list and causing exit_tasks_rcu_stop() to remove the
current task from whatever list it is on.  These lists will be used to
track tasks that are exiting, while still accounting for any RCU-tasks
quiescent states that these tasks pass though.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Link: https://lore.kernel.org/all/20240118021842.290665-1-chenzhongjin@xxxxxxxxxx/

Reported-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
Reported-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
Tested-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
---
 kernel/rcu/tasks.h | 43 +++++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 4a5d562e3189..68a8adf7de8e 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1151,25 +1151,48 @@ struct task_struct *get_rcu_tasks_gp_kthread(void)
 EXPORT_SYMBOL_GPL(get_rcu_tasks_gp_kthread);
 
 /*
- * Contribute to protect against tasklist scan blind spot while the
- * task is exiting and may be removed from the tasklist. See
- * corresponding synchronize_srcu() for further details.
+ * Protect against tasklist scan blind spot while the task is exiting and
+ * may be removed from the tasklist.  Do this by adding the task to yet
+ * another list.
+ *
+ * Note that the task will remove itself from this list, so there is no
+ * need for get_task_struct(), except in the case where rcu_tasks_pertask()
+ * adds it to the holdout list, in which case rcu_tasks_pertask() supplies
+ * the needed get_task_struct().
  */
-void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_start(void)
 {
-	current->rcu_tasks_idx = __srcu_read_lock(&tasks_rcu_exit_srcu);
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
+	struct task_struct *t = current;
+
+	WARN_ON_ONCE(!list_empty(&t->rcu_tasks_exit_list));
+	preempt_disable();
+	rtpcp = this_cpu_ptr(rcu_tasks.rtpcpu);
+	t->rcu_tasks_exit_cpu = smp_processor_id();
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	if (!rtpcp->rtp_exit_list.next)
+		INIT_LIST_HEAD(&rtpcp->rtp_exit_list);
+	list_add(&t->rcu_tasks_exit_list, &rtpcp->rtp_exit_list);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
+	preempt_enable();
 }
 
 /*
- * Contribute to protect against tasklist scan blind spot while the
- * task is exiting and may be removed from the tasklist. See
- * corresponding synchronize_srcu() for further details.
+ * Remove the task from the "yet another list" because do_exit() is now
+ * non-preemptible, allowing synchronize_rcu() to wait beyond this point.
  */
-void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_stop(void)
 {
+	unsigned long flags;
+	struct rcu_tasks_percpu *rtpcp;
 	struct task_struct *t = current;
 
-	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
+	WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
+	rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
+	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
+	list_del_init(&t->rcu_tasks_exit_list);
+	raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags);
 }
 
 /*
-- 
2.43.0