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Date: Thu, 22 Feb 2024 17:54:48 +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>,
	Ingo Molnar <mingo@redhat.com>,
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	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Joel Fernandes <joel@joelfernandes.org>,
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	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang1211@gmail.com>
Subject: Re: [PATCH v2 2/6] rcu-tasks: Add data to eliminate
 RCU-tasks/do_exit() deadlocks
Message-ID: <Zdd8WED095H8VsbY@localhost.localdomain>
References: <20240217012745.3446231-1-boqun.feng@gmail.com>
 <20240217012745.3446231-3-boqun.feng@gmail.com>
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Le Fri, Feb 16, 2024 at 05:27:37PM -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 adds the data structures that will be needed
> to rely on these quiescent states and to eliminate these deadlocks.
> 
> 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>