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

Paul E. McKenney
2023-12-11T21:39:45Z

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