From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Jonathan Corbet <corbet@lwn.net>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
linux-doc@vger.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
Randy Dunlap <rdunlap@infradead.org>,
rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
Will Deacon <will@kernel.org>
Subject: Re: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already
Date: Thu, 1 Oct 2020 21:39:50 -0700 [thread overview]
Message-ID: <20201002043950.GG29330@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200929192928.3749502-1-joel@joelfernandes.org>
On Tue, Sep 29, 2020 at 03:29:27PM -0400, Joel Fernandes (Google) wrote:
> Currently, rcu_cpu_starting() checks to see if the RCU core expects a
> quiescent state from the incoming CPU. However, the current interaction
> between RCU quiescent-state reporting and CPU-hotplug operations should
> mean that the incoming CPU never needs to report a quiescent state.
> First, the outgoing CPU reports a quiescent state if needed. Second,
> the race where the CPU is leaving just as RCU is initializing a new
> grace period is handled by an explicit check for this condition. Third,
> the CPU's leaf rcu_node structure's ->lock serializes these checks.
>
> This means that if rcu_cpu_starting() ever feels the need to report
> a quiescent state, then there is a bug somewhere in the CPU hotplug
> code or the RCU grace-period handling code. This commit therefore
> adds a WARN_ON_ONCE() to bring that bug to everyone's attention.
>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Queued for testing and further review, thank you!
Thanx, Paul
> ---
> kernel/rcu/tree.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 55d3700dd1e7..5efe0a98ea45 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4119,7 +4119,9 @@ void rcu_cpu_starting(unsigned int cpu)
> rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
> rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
> rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> - if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
> +
> + /* An incoming CPU should never be blocking a grace period. */
> + if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */
> rcu_disable_urgency_upon_qs(rdp);
> /* Report QS -after- changing ->qsmaskinitnext! */
> rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> --
> 2.28.0.709.gb0816b6eb0-goog
>
next prev parent reply other threads:[~2020-10-02 4:39 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-29 19:29 [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already Joel Fernandes (Google)
2020-09-29 19:29 ` [PATCH 2/2] docs: Update RCU's hotplug requirements with a bit about design Joel Fernandes (Google)
2020-09-29 19:32 ` Joel Fernandes
2020-10-02 19:34 ` Paul E. McKenney
2020-10-02 23:14 ` Joel Fernandes
2020-10-02 4:39 ` Paul E. McKenney [this message]
-- strict thread matches above, loose matches on Subject: below --
2020-07-30 3:02 [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already Joel Fernandes (Google)
2020-07-30 16:21 ` Paul E. McKenney
2020-07-31 1:08 ` Joel Fernandes
2020-07-31 1:42 ` Joel Fernandes
2020-07-31 3:48 ` Paul E. McKenney
2020-08-07 15:37 ` Joel Fernandes
2020-08-07 15:45 ` Joel Fernandes
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