linux-kernel - Re: [patch 14/19] softirq: Make softirq control and processing RT aware

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Message-ID: <87o8jsnop5.fsf@nanos.tec.linutronix.de>
Date:   Fri, 20 Nov 2020 14:27:18 +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>,
        Arnd Bergmann <arnd@...db.de>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, linux-parisc@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, linux-sh@...r.kernel.org,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        linux-um@...ts.infradead.org, Russell King <linux@...linux.org.uk>,
        Marc Zyngier <maz@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch 14/19] softirq: Make softirq control and processing RT aware

On Fri, Nov 20 2020 at 01:26, Frederic Weisbecker wrote:
> On Fri, Nov 13, 2020 at 03:02:21PM +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);
>> +			rcu_read_lock();
>
> Ah you lock RCU because local_bh_disable() implies it and
> since it doesn't disable preemption anymore, you must do it
> explicitly?
>
> Perhaps local_lock() should itself imply rcu_read_lock() ?

It's really only required for local_bh_disable(). Lemme add a comment.

>> +		} else {
>> +			DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt));
>> +		}
>> +	}
>> +
>> +	preempt_disable();
>
> Do you really need to disable preemption here? Migration is disabled by local_lock()
> and I can't figure out a scenario where the below can conflict with a
> preempting task.

Indeed it's pointless.

>> +	/*
>> +	 * 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);
>
> __this_cpu_add_return() ?

Yep.

Thanks,

        tglx