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From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Paul McKenney <paulmck@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@kernel.org>,
	Will Deacon <will@kernel.org>
Subject: Re: [patch V2 4/9] softirq: Make softirq control and processing RT aware
Date: Wed, 09 Dec 2020 14:30:13 +0100	[thread overview]
Message-ID: <87360f150a.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201209124233.GM2414@hirez.programming.kicks-ass.net>

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@linutronix.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

  reply	other threads:[~2020-12-09 13:30 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-04 17:01 [patch V2 0/9] softirq: Make it RT aware Thomas Gleixner
2020-12-04 17:01 ` [patch V2 1/9] softirq: Add RT specific softirq accounting Thomas Gleixner
2020-12-07 13:06   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 2/9] irqtime: Make accounting correct on RT Thomas Gleixner
2020-12-07  0:23   ` Frederic Weisbecker
2020-12-07  0:57     ` Thomas Gleixner
2020-12-07  1:14       ` Frederic Weisbecker
2020-12-07 13:27   ` Frederic Weisbecker
2020-12-07 14:44     ` Thomas Gleixner
2020-12-04 17:01 ` [patch V2 3/9] softirq: Move various protections into inline helpers Thomas Gleixner
2020-12-07 13:37   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 4/9] softirq: Make softirq control and processing RT aware Thomas Gleixner
2020-12-07 14:16   ` Frederic Weisbecker
2020-12-07 15:08     ` Thomas Gleixner
2020-12-08  0:08   ` Frederic Weisbecker
2020-12-09 10:11   ` Peter Zijlstra
2020-12-09 12:36     ` Thomas Gleixner
2020-12-09 12:42       ` Peter Zijlstra
2020-12-09 13:30         ` Thomas Gleixner [this message]
2020-12-09 10:34   ` Peter Zijlstra
2020-12-04 17:01 ` [patch V2 5/9] tick/sched: Prevent false positive softirq pending warnings on RT Thomas Gleixner
2020-12-08 12:23   ` Frederic Weisbecker
2020-12-04 17:01 ` [patch V2 6/9] rcu: Prevent false positive softirq warning " Thomas Gleixner
2020-12-04 17:59   ` Paul E. McKenney
2020-12-04 17:01 ` [patch V2 7/9] softirq: Replace barrier() with cpu_relax() in tasklet_unlock_wait() Thomas Gleixner
2020-12-07 11:39   ` Peter Zijlstra
2020-12-07 15:21     ` Thomas Gleixner
2020-12-04 17:01 ` [patch V2 8/9] tasklets: Use static inlines for stub implementations Thomas Gleixner
2020-12-04 17:02 ` [patch V2 9/9] tasklets: Prevent kill/unlock_wait deadlock on RT Thomas Gleixner
2020-12-07 11:47   ` Peter Zijlstra
2020-12-07 14:00     ` Sebastian Andrzej Siewior
2020-12-07 14:27       ` Peter Zijlstra
2020-12-07 17:55         ` Thomas Gleixner
2020-12-07 15:22       ` Thomas Gleixner
2020-12-07 15:39         ` Sebastian Andrzej Siewior
2020-12-07 17:49           ` Thomas Gleixner
2020-12-07 17:50             ` Thomas Gleixner
2020-12-06 10:05 ` [patch V2 0/9] softirq: Make it RT aware Sebastian Andrzej Siewior

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