linux-kernel - Re: [PATCH v2 2/6] rcu-tasks: Add data to eliminate RCU-tasks/do

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Date: Thu, 22 Feb 2024 12:46:04 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org, Neeraj Upadhyay <Neeraj.Upadhyay@....com>,
	Chen Zhongjin <chenzhongjin@...wei.com>,
	Yang Jihong <yangjihong1@...wei.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 2/6] rcu-tasks: Add data to eliminate
 RCU-tasks/do_exit() deadlocks

On Thu, Feb 22, 2024 at 05:54:48PM +0100, Frederic Weisbecker wrote:
> Le Fri, Feb 16, 2024 at 05:27:37PM -0800, Boqun Feng a écrit :
> > From: "Paul E. McKenney" <paulmck@...nel.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@...wei.com>
> > Reported-by: Yang Jihong <yangjihong1@...wei.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > Tested-by: Yang Jihong <yangjihong1@...wei.com>
> > Tested-by: Chen Zhongjin <chenzhongjin@...wei.com>
> > Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> 
> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thank you, I have recorded your three review tags.

							Thanx, Paul