From: "Paul E. McKenney" <paulmck@kernel.org>
To: Lai Jiangshan <laijs@linux.alibaba.com>
Cc: linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
rcu@vger.kernel.org
Subject: Re: [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP
Date: Thu, 31 Oct 2019 06:43:51 -0700 [thread overview]
Message-ID: <20191031134351.GO20975@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20191031100806.1326-2-laijs@linux.alibaba.com>
On Thu, Oct 31, 2019 at 10:07:56AM +0000, Lai Jiangshan wrote:
> If exp_deferred_qs is incorrectly set and leaked to the next
> exp GP, it may cause the next GP to be incorrectly prematurely
> completed.
Could you please provide the sequence of events leading to a such a
failure?
Also, did you provoke such a failure in testing? If so, an upgrade
to rcutorture would be good, so please tell me what you did to make
the failure happen.
I do like the reduction in state space, but I am a bit concerned about
the potential increase in contention on rnp->lock. Thoughts?
Thanx, Paul
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> kernel/rcu/tree_exp.h | 23 ++++++++++++++---------
> 1 file changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index a0e1e51c51c2..6dec21909b30 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -603,6 +603,18 @@ static void rcu_exp_handler(void *unused)
> struct rcu_node *rnp = rdp->mynode;
> struct task_struct *t = current;
>
> + /*
> + * Note that there is a large group of race conditions that
> + * can have caused this quiescent state to already have been
> + * reported, so we really do need to check ->expmask first.
> + */
> + raw_spin_lock_irqsave_rcu_node(rnp, flags);
> + if (!(rnp->expmask & rdp->grpmask)) {
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> + return;
> + }
> + raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> +
> /*
> * First, the common case of not being in an RCU read-side
> * critical section. If also enabled or idle, immediately
> @@ -628,17 +640,10 @@ static void rcu_exp_handler(void *unused)
> * a future context switch. Either way, if the expedited
> * grace period is still waiting on this CPU, set ->deferred_qs
> * so that the eventual quiescent state will be reported.
> - * Note that there is a large group of race conditions that
> - * can have caused this quiescent state to already have been
> - * reported, so we really do need to check ->expmask.
> */
> if (t->rcu_read_lock_nesting > 0) {
> - raw_spin_lock_irqsave_rcu_node(rnp, flags);
> - if (rnp->expmask & rdp->grpmask) {
> - rdp->exp_deferred_qs = true;
> - t->rcu_read_unlock_special.b.exp_hint = true;
> - }
> - raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> + rdp->exp_deferred_qs = true;
> + WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> return;
> }
>
> --
> 2.20.1
>
next prev parent reply other threads:[~2019-10-31 13:43 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-31 10:07 [PATCH 00/11] rcu: introduce percpu rcu_preempt_depth Lai Jiangshan
2019-10-31 10:07 ` [PATCH 01/11] rcu: avoid leaking exp_deferred_qs into next GP Lai Jiangshan
2019-10-31 13:43 ` Paul E. McKenney [this message]
2019-10-31 18:19 ` Lai Jiangshan
2019-10-31 19:00 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 02/11] rcu: fix bug when rcu_exp_handler() in nested interrupt Lai Jiangshan
2019-10-31 13:47 ` Paul E. McKenney
2019-10-31 14:20 ` Lai Jiangshan
2019-10-31 14:31 ` Paul E. McKenney
2019-10-31 15:14 ` Lai Jiangshan
2019-10-31 18:52 ` Paul E. McKenney
2019-11-01 0:19 ` Boqun Feng
2019-11-01 2:29 ` Lai Jiangshan
2019-10-31 10:07 ` [PATCH 03/11] rcu: clean up rcu_preempt_deferred_qs_irqrestore() Lai Jiangshan
2019-10-31 13:52 ` Paul E. McKenney
2019-10-31 15:25 ` Lai Jiangshan
2019-10-31 18:57 ` Paul E. McKenney
2019-10-31 19:02 ` Paul E. McKenney
2019-10-31 10:07 ` [PATCH 04/11] rcu: cleanup rcu_preempt_deferred_qs() Lai Jiangshan
2019-10-31 14:10 ` Paul E. McKenney
2019-10-31 14:35 ` Lai Jiangshan
2019-10-31 15:07 ` Paul E. McKenney
2019-10-31 18:33 ` Lai Jiangshan
2019-10-31 22:45 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 05/11] rcu: clean all rcu_read_unlock_special after report qs Lai Jiangshan
2019-11-01 11:54 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 06/11] rcu: clear t->rcu_read_unlock_special in one go Lai Jiangshan
2019-11-01 12:10 ` Paul E. McKenney
2019-11-01 16:58 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 07/11] rcu: set special.b.deferred_qs before wake_up() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 08/11] rcu: don't use negative ->rcu_read_lock_nesting Lai Jiangshan
2019-11-01 12:33 ` Paul E. McKenney
2019-11-16 13:04 ` Lai Jiangshan
2019-11-17 21:53 ` Paul E. McKenney
2019-11-18 1:54 ` Lai Jiangshan
2019-11-18 14:57 ` Paul E. McKenney
2019-10-31 10:08 ` [PATCH 09/11] rcu: wrap usages of rcu_read_lock_nesting Lai Jiangshan
2019-10-31 10:08 ` [PATCH 10/11] rcu: clear the special.b.need_qs in rcu_note_context_switch() Lai Jiangshan
2019-10-31 10:08 ` [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth Lai Jiangshan
2019-11-01 12:58 ` Paul E. McKenney
2019-11-01 13:13 ` Peter Zijlstra
2019-11-01 14:30 ` Paul E. McKenney
2019-11-01 15:32 ` Lai Jiangshan
2019-11-01 16:21 ` Paul E. McKenney
2019-11-01 15:47 ` Lai Jiangshan
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