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

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Date:   Wed, 09 Dec 2020 14:30:13 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paul McKenney <paulmck@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>
Subject: Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware

On Wed, Dec 09 2020 at 13:42, Peter Zijlstra wrote:
> On Wed, Dec 09, 2020 at 01:36:54PM +0100, Thomas Gleixner wrote:
>> On Wed, Dec 09 2020 at 11:11, Peter Zijlstra wrote:
>> > On Fri, Dec 04, 2020 at 06:01:55PM +0100, Thomas Gleixner wrote:
>> >> From: Thomas Gleixner <tglx@...utronix.de>
>> >> +	/* 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.
>> 
>> I know, but making this work is trying to square the circle.
>
> :-)
>
>> Any approach we tried before going this way had worse problems than
>> this particular limitation.
>
> OK, but that would've been very good Changelog material methinks.

Let me add that.

> Also, then we should probably make sure PREEMPT_RT=n builds start
> suffering the same problem by adding the local_lock unconditionally,
> otherwise this keeps being a PREEMPT_RT special and we'll keep having to
> fix it up.

For lockdep builds I assume. I'd like to postpone that for a while like
we postponed the rawlock nesting lockdep stuff until we have the vast
majority sorted out.

Thanks,

        tglx