From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
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, 9 Dec 2020 13:42:33 +0100 [thread overview]
Message-ID: <20201209124233.GM2414@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <878sa717h5.fsf@nanos.tec.linutronix.de>
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.
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.
next prev parent reply other threads:[~2020-12-09 12:43 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 [this message]
2020-12-09 13:30 ` Thomas Gleixner
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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