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Date: Fri, 23 Feb 2024 12:39:44 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>, linux-kernel@vger.kernel.org,
	rcu@vger.kernel.org, Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>,
	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>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kent Overstreet <kent.overstreet@linux.dev>,
	Oleg Nesterov <oleg@redhat.com>, Heiko Carstens <hca@linux.ibm.com>,
	Christian Brauner <brauner@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Mike Christie <michael.christie@oracle.com>,
	Mateusz Guzik <mjguzik@gmail.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Peng Zhang <zhangpeng.00@bytedance.com>
Subject: Re: [PATCH v2 3/6] rcu-tasks: Initialize data to eliminate
 RCU-tasks/do_exit() deadlocks
Message-ID: <ZdiEAPczZT+GVuxa@lothringen>
References: <20240217012745.3446231-1-boqun.feng@gmail.com>
 <20240217012745.3446231-4-boqun.feng@gmail.com>
 <Zdd0b3HI4uNAoc2P@localhost.localdomain>
 <1e265c80-2008-4419-8a21-d05c39e1e53e@paulmck-laptop>
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In-Reply-To: <1e265c80-2008-4419-8a21-d05c39e1e53e@paulmck-laptop>

On Thu, Feb 22, 2024 at 12:41:55PM -0800, Paul E. McKenney wrote:
> On Thu, Feb 22, 2024 at 05:21:03PM +0100, Frederic Weisbecker wrote:
> > Le Fri, Feb 16, 2024 at 05:27:38PM -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 initializes 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>

Looks good, thanks!

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