linux-kernel - Re: [PATCH 0/8] rcu: Fix expedited GP deadlock (and cleanup some nocb stuff)

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Message-ID: <c429620e-53ab-4cd1-b1c2-ca83f4f3cea7@paulmck-laptop>
Date:   Mon, 11 Dec 2023 13:39:40 -0800
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 <joel@...lfernandes.org>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        Uladzislau Rezki <urezki@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 0/8] rcu: Fix expedited GP deadlock (and cleanup some
 nocb stuff)

On Mon, Dec 11, 2023 at 09:04:04PM +0100, Frederic Weisbecker wrote:
> Le Mon, Dec 11, 2023 at 08:38:59AM -0800, Paul E. McKenney a écrit :
> > On Fri, Dec 08, 2023 at 11:05:37PM +0100, Frederic Weisbecker wrote:
> > > TREE04 can trigger a writer stall if run with memory pressure. This
> > > is due to a circular dependency between waiting for expedited grace
> > > period and polling on expedited grace period when workqueues go back
> > > to mayday serialization.
> > > 
> > > Here is a proposal fix.
> > 
> > The torture.sh "acceptance test" with KCSAN and --duration 30 ran
> > fine except for this in TREE09:
> > 
> > kernel/rcu/tree_nocb.h:1785:13: error: unused function '__call_rcu_nocb_wake' [-Werror,-Wunused-function]
> > 
> > My guess is that the declaration of __call_rcu_nocb_wake() in
> > kernel/rcu/tree.h needs an "#ifdef CONFIG_SMP", but you might have a
> > better fix.
> 
> Could be because if CONFIG_RCU_NO_CB_CPU=n, the function is only called
> (though as dead code) from rcutree_migrate_callbacks() which in turn only
> exists if CONFIG_HOTPLUG_CPU=y.
> 
> Something like that then:
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 35f7af331e6c..e1ff53d5084c 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -445,6 +445,8 @@ static void rcu_qs(void);
>  static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
>  #ifdef CONFIG_HOTPLUG_CPU
>  static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
> +static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
> +				 unsigned long flags);
>  #endif /* #ifdef CONFIG_HOTPLUG_CPU */
>  static int rcu_print_task_exp_stall(struct rcu_node *rnp);
>  static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
> @@ -466,8 +468,6 @@ static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>  				  unsigned long j, bool lazy);
>  static void call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *head,
>  			  rcu_callback_t func, unsigned long flags, bool lazy);
> -static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
> -				 unsigned long flags);
>  static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
>  static bool do_nocb_deferred_wakeup(struct rcu_data *rdp);
>  static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);

This one passes TREE01 and TINY01, but on TREE09 still gets this:

kernel/rcu/tree_nocb.h:1785:13: error: ‘__call_rcu_nocb_wake’ defined but not used [-Werror=unused-function]

Huh.  I suppose that there is always __maybe_unused?

							Thanx, Paul