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

[Peter Zijlstra]

On Fri, Dec 04, 2020 at 06:01:55PM +0100, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Provide a local lock based serialization for soft interrupts on RT which
> allows the local_bh_disabled() sections and servicing soft interrupts to be
> preemptible.
> 
> Provide the necessary inline helpers which allow to reuse the bulk of the
> softirq processing code.

> +struct softirq_ctrl {
> +	local_lock_t	lock;
> +	int		cnt;
> +};
> +
> +static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
> +	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
> +};
> +
> +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);

AFAICT this significantly changes the locking rules.

Where previously we could do:

	spin_lock(&ponies)
	spin_lock_bh(&foo);

vs

	spin_lock_bh(&bar);
	spin_lock(&ponies)

provided the rest of the code observed: bar -> ponies -> foo
and never takes ponies from in-softirq.

This is now a genuine deadlock on RT.

Also see:

  https://lkml.kernel.org/r/X9CheYjuXWc75Spa@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> +			/* Required to meet the RCU bottomhalf requirements. */
> +			rcu_read_lock();
> +		} else {
> +			DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
> +		}
> +	}
> +
> +	/*
> +	 * 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);
> +	}
> +}