Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware

[Frederic Weisbecker]

On Fri, Dec 04, 2020 at 06:01:55PM +0100, Thomas Gleixner wrote:
> +void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
> +{
> +	unsigned long flags;
> +	int newcnt;
> +
> +	WARN_ON_ONCE(in_hardirq());
> +
> +	/* First entry of a task into a BH disabled section? */
> +	if (!current->softirq_disable_cnt) {
> +		if (preemptible()) {
> +			local_lock(&softirq_ctrl.lock);
> +			/* Required to meet the RCU bottomhalf requirements. */
> +			rcu_read_lock();
> +		} else {
> +			DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));

So, to be clear this adds a new constraint where we can't call
local_bh_disable() inside a preempt disabled section? I guess the rest of the
RT code chased all the new offenders :-)

> +		}
> +	}
> +
> +	/*
> +	 * Track the per CPU softirq disabled state. On RT this is per CPU
> +	 * state to allow preemption of bottom half disabled sections.
> +	 */
> +	newcnt = __this_cpu_add_return(softirq_ctrl.cnt, cnt);
> +	/*
> +	 * Reflect the result in the task state to prevent recursion on the
> +	 * local lock and to make softirq_count() & al work.
> +	 */
> +	current->softirq_disable_cnt = newcnt;
> +
> +	if (IS_ENABLED(CONFIG_TRACE_IRQFLAGS) && newcnt == cnt) {
> +		raw_local_irq_save(flags);
> +		lockdep_softirqs_off(ip);
> +		raw_local_irq_restore(flags);
> +	}
> +}
> +EXPORT_SYMBOL(__local_bh_disable_ip);
> +
[...]
> +
> +void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
> +{
> +	bool preempt_on = preemptible();
> +	unsigned long flags;
> +	u32 pending;
> +	int curcnt;
> +
> +	WARN_ON_ONCE(in_irq());
> +	lockdep_assert_irqs_enabled();
> +
> +	local_irq_save(flags);
> +	curcnt = this_cpu_read(softirq_ctrl.cnt);

__this_cpu_read() ?

> +
> +	/*
> +	 * If this is not reenabling soft interrupts, no point in trying to
> +	 * run pending ones.
> +	 */
> +	if (curcnt != cnt)
> +		goto out;

I guess you could move the local_irq_save() here?

> +	pending = local_softirq_pending();
> +	if (!pending || ksoftirqd_running(pending))
> +		goto out;
> +
> +	/*
> +	 * If this was called from non preemptible context, wake up the
> +	 * softirq daemon.
> +	 */
> +	if (!preempt_on) {
> +		wakeup_softirqd();
> +		goto out;
> +	}
> +
> +	/*
> +	 * Adjust softirq count to SOFTIRQ_OFFSET which makes
> +	 * in_serving_softirq() become true.
> +	 */
> +	cnt = SOFTIRQ_OFFSET;
> +	__local_bh_enable(cnt, false);
> +	__do_softirq();
> +
> +out:
> +	__local_bh_enable(cnt, preempt_on);
> +	local_irq_restore(flags);
> +}
> +EXPORT_SYMBOL(__local_bh_enable_ip);

Thanks.