linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Message-ID: <20140801141144.GA30293@redhat.com>
Date:	Fri, 1 Aug 2014 16:11:44 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On 07/31, Paul E. McKenney wrote:
>
> +/* Lists of tasks that we are still waiting for during this grace period. */
> +static LIST_HEAD(rcu_tasks_holdouts);

This can be local var in rcu_tasks_kthread()

> +		while (!list_empty(&rcu_tasks_holdouts)) {
> +			schedule_timeout_interruptible(HZ / 10);
> +			flush_signals(current);

Still can't undestand why your paranoia wants flush_signals ;)
This is unneeded and confusing. If you think we can have a bug here,
then we should ot hide it, WARN_ON(signal_pending) would be better.
And if you think signal_pending(current) is possible, why do you
check this only after schedule_interruptible() ?

> +		synchronize_sched();
> +
> +		/* Invoke the callbacks. */
> +		while (list) {
> +			next = list->next;
> +			local_bh_disable();
> +			list->func(list);
> +			local_bh_enable();
> +			list = next;
> +			cond_resched();
> +		}

Not sure this makes any sense, but perhaps we can check for the new
callbacks and start the next gp. IOW, the main loop roughly does

	for (;;) {
		list = rcu_tasks_cbs_head;
		rcu_tasks_cbs_head = NULL;

		if (!list)
			sleep();

		synchronize_sched();

		wait_for_rcu_tasks_holdout();

		synchronize_sched();

		process_callbacks(list);
	}

we can "join" 2 synchronize_sched's and do

	ready_list = NULL;
	for (;;) {
		list = rcu_tasks_cbs_head;
		rcu_tasks_cbs_head = NULL;

		if (!list && !ready_list)
			sleep();

		synchronize_sched();

		if (ready_list) {
			process_callbacks(ready_list);
			ready_list = NULL;
		}

		if (!list)
			continue;

		wait_for_rcu_tasks_holdout();
		ready_list = list;
	}

Oleg.

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