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Date: Fri, 28 Jun 2019 11:52:19 -0700 From: "Paul E. McKenney" <paulmck@...ux.ibm.com> To: Sebastian Andrzej Siewior <bigeasy@...utronix.de> Cc: Peter Zijlstra <peterz@...radead.org>, Joel Fernandes <joel@...lfernandes.org>, Steven Rostedt <rostedt@...dmis.org>, rcu <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Josh Triplett <josh@...htriplett.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Lai Jiangshan <jiangshanlai@...il.com> Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs On Fri, Jun 28, 2019 at 08:40:26PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-06-28 08:30:50 [-0700], Paul E. McKenney wrote: > > On Fri, Jun 28, 2019 at 03:54:33PM +0200, Peter Zijlstra wrote: > > > On Thu, Jun 27, 2019 at 11:41:07AM -0700, Paul E. McKenney wrote: > > > > Or just don't do the wakeup at all, if it comes to that. I don't know > > > > of any way to determine whether rcu_read_unlock() is being called from > > > > the scheduler, but it has been some time since I asked Peter Zijlstra > > > > about that. > > > > > > There (still) is no 'in-scheduler' state. > > > > Well, my TREE03 + threadirqs rcutorture test ran for ten hours last > > night with no problems, so we just might be OK. > > > > The apparent fix is below, though my approach would be to do backports > > for the full set of related changes. > > > > Joel, Sebastian, how goes any testing from your end? Any reason > > to believe that this does not represent a fix? (Me, I am still > > concerned about doing raise_softirq() from within a threaded > > interrupt, but am not seeing failures.) > > For some reason it does not trigger as good as it did yesterday. I swear that I wasn't watching!!! ;-) But I do know that feeling. > Commit > - 23634ebc1d946 ("rcu: Check for wakeup-safe conditions in > rcu_read_unlock_special()") does not trigger the bug within 94 > attempts. > > - 48d07c04b4cc1 ("rcu: Enable elimination of Tree-RCU softirq > processing") needed 12 attempts to trigger the bug. That matches my belief that 23634ebc1d946 ("rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()") will at least greatly decrease the probability of this bug occurring. Thanx, Paul
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