From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: "Zhang, Qiang" <Qiang.Zhang@windriver.com>,
Uladzislau Rezki <urezki@gmail.com>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>, rcu <rcu@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: RCU: Question rcu_preempt_blocked_readers_cgp in rcu_gp_fqs_loop func
Date: Mon, 14 Sep 2020 16:06:36 -0400 [thread overview]
Message-ID: <20200914200636.GB2579423@google.com> (raw)
In-Reply-To: <20200909112241.GE29330@paulmck-ThinkPad-P72>
On Wed, Sep 09, 2020 at 04:22:41AM -0700, Paul E. McKenney wrote:
> On Wed, Sep 09, 2020 at 07:03:39AM +0000, Zhang, Qiang wrote:
> >
> > When config preempt RCU, and then there are multiple levels node, the current task is preempted in rcu read critical region.
> > the current task be add to "rnp->blkd_tasks" link list, and the "rnp->gp_tasks" may be assigned a value . these rnp is leaf node in RCU tree.
> >
> > But in "rcu_gp_fqs_loop" func, we check blocked readers in root node.
> >
> > static void rcu_gp_fqs_loop(void)
> > {
> > .....
> > struct rcu_node *rnp = rcu_get_root();
> > .....
> > if (!READ_ONCE(rnp->qsmask) &&
> > !rcu_preempt_blocked_readers_cgp(rnp)) ------> rnp is root node
> > break;
> > ....
> > }
> >
> > the root node's blkd_tasks never add task, the "rnp->gp_tasks" is never be assigned value, this check is invailed.
> > Should we check leaf nodes like this
>
> There are two cases:
>
> 1. There is only a single rcu_node structure, which is both root
> and leaf. In this case, the current check is required: Both
> ->qsmask and the ->blkd_tasks list must be checked. Your
> rcu_preempt_blocked_readers() would work in this case, but
> the current code is a bit faster because it does not need
> to acquire the ->lock nor does it need the loop overhead.
>
> 2. There are multiple levels. In this case, as you say, the root
> rcu_node structure's ->blkd_tasks list will always be empty.
> But also in this case, the root rcu_node structure's ->qsmask
> cannot be zero until all the leaf rcu_node structures' ->qsmask
> fields are zero and their ->blkd_tasks lists no longer have
> tasks blocking the current grace period. This means that your
> rcu_preempt_blocked_readers() function would never return
> true in this case.
>
> So the current code is fine.
>
> Are you seeing failures on mainline kernels? If so, what is the failure
> mode?
Nice question! He had me wondering there too for a bit ;-)
Thanks,
- Joel
next prev parent reply other threads:[~2020-09-14 20:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-14 6:45 [PATCH] rcu: shrink each possible cpu krcp qiang.zhang
2020-08-14 18:51 ` Uladzislau Rezki
2020-08-17 22:03 ` Joel Fernandes
2020-08-18 17:18 ` Paul E. McKenney
2020-08-18 19:00 ` Joel Fernandes
2020-08-18 21:03 ` Paul E. McKenney
2020-08-18 21:55 ` Uladzislau Rezki
2020-08-18 22:02 ` Paul E. McKenney
2020-08-19 0:04 ` Joel Fernandes
2020-08-19 3:00 ` 回复: " Zhang, Qiang
2020-08-19 13:04 ` Paul E. McKenney
2020-08-19 13:56 ` Joel Fernandes
2020-08-19 15:21 ` Paul E. McKenney
2020-08-19 15:54 ` Joel Fernandes
2020-08-19 15:58 ` Uladzislau Rezki
2020-08-20 22:39 ` Joel Fernandes
2020-08-21 15:33 ` Paul E. McKenney
2020-08-31 9:30 ` Uladzislau Rezki
2020-09-09 6:35 ` Zhang, Qiang
2020-09-09 7:03 ` RCU: Question rcu_preempt_blocked_readers_cgp in rcu_gp_fqs_loop func Zhang, Qiang
2020-09-09 11:22 ` Paul E. McKenney
2020-09-10 3:25 ` 回复: " Zhang, Qiang
2020-09-14 20:06 ` Joel Fernandes [this message]
2020-08-19 11:22 ` [PATCH] rcu: shrink each possible cpu krcp Uladzislau Rezki
2020-08-19 13:25 ` Joel Fernandes
2020-08-18 23:25 ` Joel Fernandes
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