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From: Joel Fernandes <joel@joelfernandes.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>, rcu <rcu@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs
Date: Fri, 28 Jun 2019 19:12:41 -0400	[thread overview]
Message-ID: <20190628231241.GA9243@google.com> (raw)
In-Reply-To: <20190628222547.GE26519@linux.ibm.com>

On Fri, Jun 28, 2019 at 03:25:47PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 28, 2019 at 05:40:18PM -0400, Joel Fernandes wrote:
> > Hi Paul,
> > 
> > On Fri, Jun 28, 2019 at 01:04:23PM -0700, Paul E. McKenney wrote:
> > [snip]
> > > > > > 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.
> > > > 
> > > > I was just typing a reply that I can't reproduce it with:
> > > >   rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()
> > > > 
> > > > I am trying to revert enough of this patch to see what would break things,
> > > > however I think a better exercise might be to understand more what the patch
> > > > does why it fixes things in the first place ;-) It is probably the
> > > > deferred_qs thing.
> > > 
> > > The deferred_qs flag is part of it!  Looking forward to hearing what
> > > you come up with as being the critical piece of this commit.
> > 
> > The new deferred_qs flag indeed saves the machine from the dead-lock.
> > 
> > If we don't want the deferred_qs, then the below patch also fixes the issue.
> > However, I am more sure than not that it does not handle all cases (such as
> > what if we previously had an expedited grace period IPI in a previous reader
> > section and had to to defer processing. Then it seems a similar deadlock
> > would present. But anyway, the below patch does fix it for me! It is based on
> > your -rcu tree commit 23634ebc1d946f19eb112d4455c1d84948875e31 (rcu: Check
> > for wakeup-safe conditions in rcu_read_unlock_special()).
> 
> The point here being that you rely on .b.blocked rather than
> .b.deferred_qs.  Hmmm...  There are a number of places that check all
> the bits via the .s leg of the rcu_special union.  The .s check in
> rcu_preempt_need_deferred_qs() should be OK because it is conditioned
> on t->rcu_read_lock_nesting of zero or negative.
> Do rest of those also work out OK?
> 
> It would be nice to remove the flag, but doing so clearly needs careful
> review and testing.

Agreed. I am planning to do an audit of this code within the next couple of
weeks so I will be on the look out for any optimization opportunities related
to this. Will let you know if this can work. For now I like your patch better
because it is more conservative and doesn't cause any space overhead.

If you'd like, please free to included my Tested-by on it:

Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>

If you had a chance, could you also point to me any tests that show
performance improvement with the irqwork patch, on the expedited GP usecase?
I'd like to try it out as well. I guess rcuperf should have some?

thanks!

 - Joel
 

  reply	other threads:[~2019-06-28 23:12 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-26 13:54 [RFC] Deadlock via recursive wakeup via RCU with threadirqs Sebastian Andrzej Siewior
2019-06-26 16:25 ` Paul E. McKenney
2019-06-27  7:47   ` Sebastian Andrzej Siewior
2019-06-27 15:52     ` Paul E. McKenney
2019-06-27 14:24   ` Joel Fernandes
2019-06-27 14:34     ` Steven Rostedt
2019-06-27 15:30       ` Joel Fernandes
2019-06-27 15:37         ` Joel Fernandes
2019-06-27 15:40           ` Sebastian Andrzej Siewior
2019-06-27 15:42             ` Joel Fernandes
2019-06-27 17:43             ` Joel Fernandes
2019-06-27 17:46               ` Joel Fernandes
2019-06-27 18:11                 ` Paul E. McKenney
2019-06-27 18:27                   ` Joel Fernandes
2019-06-27 18:51                     ` Joel Fernandes
2019-06-27 19:14                       ` Paul E. McKenney
2019-06-27 19:15                     ` Paul E. McKenney
2019-06-27 18:30                   ` Paul E. McKenney
2019-06-27 20:45                     ` Paul E. McKenney
2019-06-27 15:55         ` Paul E. McKenney
2019-06-27 16:47           ` Joel Fernandes
2019-06-27 17:38             ` Paul E. McKenney
2019-06-27 18:16               ` Joel Fernandes
2019-06-27 18:41                 ` Paul E. McKenney
2019-06-27 20:17                   ` Scott Wood
2019-06-27 20:36                     ` Paul E. McKenney
2019-06-28  7:31                       ` Byungchul Park
2019-06-28  7:43                         ` Byungchul Park
2019-06-28  8:14                           ` Byungchul Park
2019-06-28  8:24                             ` Byungchul Park
2019-06-28 12:24                               ` Paul E. McKenney
2019-06-28  9:10                           ` Byungchul Park
2019-06-28  9:28                             ` Byungchul Park
2019-06-28 12:21                           ` Paul E. McKenney
2019-06-28 10:40                         ` Byungchul Park
2019-06-28 12:27                           ` Paul E. McKenney
2019-06-28 15:44                           ` Steven Rostedt
2019-06-29 15:12                             ` Andrea Parri
2019-06-29 16:55                               ` Paul E. McKenney
2019-06-29 18:09                                 ` Andrea Parri
2019-06-29 18:21                                   ` Andrea Parri
2019-06-29 19:15                                   ` Paul E. McKenney
2019-06-29 19:35                                     ` Andrea Parri
2019-06-30 23:55                             ` Byungchul Park
2019-06-28 14:15                       ` Peter Zijlstra
2019-06-28 15:54                         ` Paul E. McKenney
2019-06-28 16:04                           ` Peter Zijlstra
2019-06-28 17:20                             ` Paul E. McKenney
2019-07-01  9:42                               ` Peter Zijlstra
2019-07-01 10:24                                 ` Sebastian Andrzej Siewior
2019-07-01 12:23                                   ` Paul E. McKenney
2019-07-01 14:00                                     ` Peter Zijlstra
2019-07-01 16:01                                       ` Paul E. McKenney
2019-06-28 20:01                         ` Scott Wood
2019-07-01  9:45                           ` Peter Zijlstra
2019-06-28 13:54                   ` Peter Zijlstra
2019-06-28 15:30                     ` Paul E. McKenney
2019-06-28 18:40                       ` Sebastian Andrzej Siewior
2019-06-28 18:52                         ` Paul E. McKenney
2019-06-28 19:24                           ` Joel Fernandes
2019-06-28 20:04                             ` Paul E. McKenney
2019-06-28 21:40                               ` Joel Fernandes
2019-06-28 22:25                                 ` Paul E. McKenney
2019-06-28 23:12                                   ` Joel Fernandes [this message]
2019-06-29  0:06                                     ` Paul E. McKenney
2019-06-28 16:40                   ` Joel Fernandes
2019-06-28 16:45                     ` Joel Fernandes
2019-06-28 17:30                       ` Paul E. McKenney
2019-06-28 17:41                         ` Paul E. McKenney
2019-06-28 17:45                         ` Sebastian Andrzej Siewior
2019-06-28 18:07                           ` Joel Fernandes
2019-06-28 18:20                             ` Sebastian Andrzej Siewior
2019-07-01  2:08                               ` Joel Fernandes
2019-06-28 18:22                           ` Paul E. McKenney
2019-06-28 19:29                             ` Joel Fernandes
2019-06-28 20:06                               ` Paul E. McKenney
2019-06-28 18:05                         ` Joel Fernandes
2019-06-28 18:23                           ` Paul E. McKenney

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