From: Frederic Weisbecker <frederic@kernel.org>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Boqun Feng <boqun.feng@gmail.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Josh Triplett <josh@joshtriplett.org>,
Stable <stable@vger.kernel.org>,
Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 01/13] rcu/nocb: Fix potential missed nocb_timer rearm
Date: Thu, 25 Feb 2021 01:48:13 +0100 [thread overview]
Message-ID: <20210225004813.GB12431@lothringen> (raw)
In-Reply-To: <20210225001425.GL2743@paulmck-ThinkPad-P72>
On Wed, Feb 24, 2021 at 04:14:25PM -0800, Paul E. McKenney wrote:
> On Wed, Feb 24, 2021 at 11:06:06PM +0100, Frederic Weisbecker wrote:
> > I managed to recollect some pieces of my brain. So keep the above but
> > let's change the point 10:
> >
> > 10. CPU 0 enqueues its second callback, this time with interrupts
> > enabled so it can wake directly ->nocb_gp_kthread.
> > It does so with calling __wake_nocb_gp() which also cancels the
> > pending timer that got queued in step 2. But that doesn't reset
> > CPU 0's ->nocb_defer_wakeup which is still set to RCU_NOCB_WAKE.
> > So CPU 0's ->nocb_defer_wakeup and CPU 0's ->nocb_timer are now
> > desynchronized.
> >
> > 11. ->nocb_gp_kthread associates the callback queued in 10 with a new
> > grace period, arrange for it to start and sleeps on it.
> >
> > 12. The grace period ends, ->nocb_gp_kthread awakens and wakes up
> > CPU 0's ->nocb_cb_kthread which invokes the callback queued in 10.
> >
> > 13. CPU 0 enqueues its third callback, this time with interrupts
> > disabled so it tries to queue a deferred wakeup. However
> > ->nocb_defer_wakeup has a stalled RCU_NOCB_WAKE value which prevents
> > the CPU 0's ->nocb_timer, that got cancelled in 10, from being armed.
> >
> > 14. CPU 0 has its pending callback and it may go unnoticed until
> > some other CPU ever wakes up ->nocb_gp_kthread or CPU 0 ever calls
> > an explicit deferred wake up caller like idle entry.
> >
> > I hope I'm not missing something this time...
>
> Thank you, that does sound plausible. I guess I can see how rcutorture
> might have missed this one!
I must admit it requires a lot of stars to be aligned :-)
Thanks.
next prev parent reply other threads:[~2021-02-25 0:49 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-23 0:09 [PATCH 00/13] rcu/nocb updates v2 Frederic Weisbecker
2021-02-23 0:09 ` [PATCH 01/13] rcu/nocb: Fix potential missed nocb_timer rearm Frederic Weisbecker
2021-02-24 18:37 ` Paul E. McKenney
2021-02-24 22:06 ` Frederic Weisbecker
2021-02-25 0:14 ` Paul E. McKenney
2021-02-25 0:48 ` Frederic Weisbecker [this message]
2021-02-25 1:07 ` Paul E. McKenney
2021-03-02 1:48 ` Paul E. McKenney
2021-03-02 12:34 ` Frederic Weisbecker
2021-03-02 18:17 ` Paul E. McKenney
2021-03-03 1:35 ` Frederic Weisbecker
2021-03-03 2:06 ` Paul E. McKenney
2021-03-03 2:17 ` Frederic Weisbecker
2021-03-03 11:15 ` Neeraj Upadhyay
2021-02-23 0:10 ` [PATCH 02/13] rcu/nocb: Disable bypass when CPU isn't completely offloaded Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 03/13] rcu/nocb: Remove stale comment above rcu_segcblist_offload() Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 04/13] rcu/nocb: Move trace_rcu_nocb_wake() calls outside nocb_lock when possible Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 05/13] rcu/nocb: Merge nocb_timer to the rdp leader Frederic Weisbecker
2021-03-03 1:15 ` [PATCH 05/13] rcu/nocb: Use the rcuog CPU's ->nocb_timer Paul E. McKenney
2021-03-10 22:05 ` Frederic Weisbecker
2021-03-16 0:02 ` Paul E. McKenney
2021-02-23 0:10 ` [PATCH 06/13] timer: Revert "timer: Add timer_curr_running()" Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 07/13] rcu/nocb: Directly call __wake_nocb_gp() from bypass timer Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 08/13] rcu/nocb: Allow de-offloading rdp leader Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 09/13] rcu/nocb: Cancel nocb_timer upon nocb_gp wakeup Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 10/13] rcu/nocb: Delete bypass_timer " Frederic Weisbecker
2021-03-03 1:24 ` Paul E. McKenney
2021-03-10 22:17 ` Frederic Weisbecker
2021-03-15 14:53 ` Boqun Feng
2021-03-15 22:56 ` Frederic Weisbecker
2021-03-16 0:02 ` Paul E. McKenney
2021-02-23 0:10 ` [PATCH 11/13] rcu/nocb: Only cancel nocb timer if not polling Frederic Weisbecker
2021-03-03 1:22 ` Paul E. McKenney
2021-03-10 22:08 ` Frederic Weisbecker
2021-02-23 0:10 ` [PATCH 12/13] rcu/nocb: Prepare for finegrained deferred wakeup Frederic Weisbecker
2021-03-16 3:02 ` Paul E. McKenney
2021-03-16 11:45 ` Frederic Weisbecker
2021-03-16 14:02 ` Paul E. McKenney
2021-02-23 0:10 ` [PATCH 13/13] rcu/nocb: Unify timers Frederic Weisbecker
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