Re: [PATCH v2 2/6] rcu-tasks: Add data to eliminate RCU-tasks/do_exit() deadlocks

Paul E. McKenney
2024-02-22T20:46:04Z

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@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>

Thank you, I have recorded your three review tags.

Thanx, Paul