[PATCH AUTOSEL 6.8 05/11] rcu-tasks: Maintain lists to eliminate RCU-tasks/do_exit() deadlocks

Sasha Levin
2024-03-24T17:06:04Z

[ Upstream commit 6b70399f9ef3809f6e308fd99dd78b072c1bd05c ]

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@huawei.com/

Reported-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reported-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Yang Jihong <yangjihong1@huawei.com>
Tested-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 4a5d562e31892..68a8adf7de8e9 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