linux-kernel - Re: [PATCH 4/5] rcu/exp: Warn on QS requested on dying CPU

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Message-ID: <476d11c8-ff63-4abc-a894-b9dabf92df8b@paulmck-laptop>
Date: Tue, 18 Mar 2025 10:21:48 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 4/5] rcu/exp: Warn on QS requested on dying CPU

On Fri, Mar 14, 2025 at 03:36:41PM +0100, Frederic Weisbecker wrote:
> It is not possible to send an IPI to a dying CPU that has passed the
> CPUHP_TEARDOWN_CPU stage. Remaining unhandled IPIs are handled later at
> CPUHP_AP_SMPCFD_DYING stage by stop machine. This is the last
> opportunity for RCU exp handler to request an expedited quiescent state.
> And the upcoming final context switch between stop machine and idle must
> have reported the requested context switch.
> 
> Therefore, it should not be possible to observe a pending requested
> expedited quiescent state when RCU finally stops watching the outgoing
> CPU. Once IPIs aren't possible anymore, the QS for the target CPU will
> be reported on its behalf by the RCU exp kworker.
> 
> Provide an assertion to verify those expectations.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

But what do we do if this assertion triggers?  And do we want it to take
effect only in kernels built with CONFIG_PROVE_RCU?  Or is such a broken
assumption bad enough to justify a splat in production kernels?

If the answer to the last question is "yes" (and you, not me, work for
a distro, so it is your question to answer):

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

							Thanx, Paul

> ---
>  kernel/rcu/tree.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3fe68057d8b4..79dced5fb72e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4321,6 +4321,12 @@ void rcutree_report_cpu_dead(void)
>  	 * may introduce a new READ-side while it is actually off the QS masks.
>  	 */
>  	lockdep_assert_irqs_disabled();
> +	/*
> +	 * CPUHP_AP_SMPCFD_DYING was the last call for rcu_exp_handler() execution.
> +	 * The requested QS must have been reported on the last context switch
> +	 * from stop machine to idle.
> +	 */
> +	WARN_ON_ONCE(rdp->cpu_no_qs.b.exp);
>  	// Do any dangling deferred wakeups.
>  	do_nocb_deferred_wakeup(rdp);
>  
> -- 
> 2.48.1
>