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

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Date:   Wed, 9 Dec 2020 11:34:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.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 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()) {

This reminds me of that migrate_disable() thing that didn't work, and
has similar problems.

	preempt_disable();
	local_bh_disable();
	preempt_enable();
	spin_lock(&foo);

is valid (albeit daft) code that will now malfunction.

> +			local_lock(&softirq_ctrl.lock);