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Date: Thu, 22 Feb 2024 17:46:22 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Chen Zhongjin <chenzhongjin@huawei.com>,
	Yang Jihong <yangjihong1@huawei.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>
Subject: Re: [PATCH v2 5/6] rcu-tasks: Eliminate deadlocks involving
 do_exit() and RCU tasks
Message-ID: <Zdd6Xm15-WfF5cus@localhost.localdomain>
References: <20240217012745.3446231-1-boqun.feng@gmail.com>
 <20240217012745.3446231-6-boqun.feng@gmail.com>
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Le Fri, Feb 16, 2024 at 05:27:40PM -0800, Boqun Feng a ?crit :
> From: "Paul E. McKenney" <paulmck@kernel.org>
> 
> Holding a mutex across synchronize_rcu_tasks() and acquiring
> that same mutex in code called from do_exit() after its call to
> exit_tasks_rcu_start() but before its call to exit_tasks_rcu_stop()
> results in deadlock.  This is by design, because tasks that are far
> enough into do_exit() are no longer present on the tasks list, making
> it a bit difficult for RCU Tasks to find them, let alone wait on them
> to do a voluntary context switch.  However, such deadlocks are becoming
> more frequent.  In addition, lockdep currently does not detect such
> deadlocks and they can be difficult to reproduce.
> 
> In addition, if a task voluntarily context switches during that time
> (for example, if it blocks acquiring a mutex), then this task is in an
> RCU Tasks quiescent state.  And with some adjustments, RCU Tasks could
> just as well take advantage of that fact.
> 
> This commit therefore eliminates these deadlock by replacing the
> SRCU-based wait for do_exit() completion with per-CPU lists of tasks
> currently exiting.  A given task will be on one of these per-CPU lists for
> the same period of time that this task would previously have been in the
> previous SRCU read-side critical section.  These lists enable RCU Tasks
> to find the tasks that have already been removed from the tasks list,
> but that must nevertheless be waited upon.
> 
> The RCU Tasks grace period gathers any of these do_exit() tasks that it
> must wait on, and adds them to the list of holdouts.  Per-CPU locking
> and get_task_struct() are used to synchronize addition to and removal
> from these lists.
> 
> 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>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>