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

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Date:   Mon, 07 Dec 2020 16:08:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware

On Mon, Dec 07 2020 at 15:16, Frederic Weisbecker wrote:
> 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 :-)

There are not that many.

>> +		}
>> +	}
>> +
>> +	/*
>> +	 * 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() ?

Yes.

>> +
>> +	/*
>> +	 * 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?

Indeed.