From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
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 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up
Date: Thu, 28 Jan 2021 13:45:24 -0800 [thread overview]
Message-ID: <20210128214524.GV2743@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20210128213413.GC122776@lothringen>
On Thu, Jan 28, 2021 at 10:34:13PM +0100, Frederic Weisbecker wrote:
> On Thu, Jan 28, 2021 at 11:12:28AM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 28, 2021 at 06:12:10PM +0100, Frederic Weisbecker wrote:
> > > Simply checking if the segcblist is enabled is enough to know if we
> > > need to initialize it or not. It's safe to check within hotplug
> > > machine.
> > >
> > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > > Cc: Joel Fernandes <joel@joelfernandes.org>
> > > Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
> > > Cc: Boqun Feng <boqun.feng@gmail.com>
> >
> > Hmmm...
> >
> > At the start of a CPU-hotplug operation, an incoming CPU's callback
> > list can be in a number of states:
> >
> > 1. Disabled and empty. This is the case when the boot CPU has
> > not done call_rcu(), when a non-boot CPU first comes online,
> > and when a non-offloaded CPU comes back online. In this case,
> > it is permissible to initialize ->cblist. Because either the
> > CPU is currently running with interrupts disabled (boot CPU)
> > or is not yet running at all (other CPUs), it is not necessary
> > to acquire ->nocb_lock.
> >
> > 2. Disabled and non-empty. This is the case when the boot CPU has
> > done call_rcu(). It is not permissible to initialize ->cblist
> > because doing so will leak any callbacks posted by early boot
> > invocations of call_rcu().
>
> I don't think that's possible. In this case __call_rcu() has called
> rcu_segcblist_init() and has enabled the segcblist.
You are right, rcu_segcblist_init() would have been called in that
case and it has: rcu_segcblist_set_flags(rsclp, SEGCBLIST_ENABLED).
> > Test for the possibility of leaking by building with
> > CONFIG_PROVE_RCU=y and booting with rcupdate.rcu_self_test=1.
> >
> > 3. Enabled, whether empty or not. This is the case when an
> > offloaded CPU comes back online. This is the only case where
> > the ->nocb_lock must be held to modify ->cblist. However,
> > it is not necessarily to modify ->cblist because the rcuoc
> > kthread is on the job.
> >
> > So I believe that it is necessary to check for both disabled and empty.
> > But don't take my word for it! Build with CONFIG_PROVE_RCU=y and boot
> > with rcupdate.rcu_self_test=1. ;-)
>
> I'm trying that :-)
Even better!
Thanx, Paul
next prev parent reply other threads:[~2021-01-28 21:46 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-28 17:12 [PATCH 00/16] rcu/nocb updates Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 01/16] rcu/nocb: Fix potential missed nocb_timer rearm Frederic Weisbecker
[not found] ` <20210128184834.GP2743@paulmck-ThinkPad-P72>
2021-01-28 21:23 ` Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 02/16] rcu/nocb: Comment the reason behind BH disablement on batch processing Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 03/16] rcu/nocb: Forbid NOCB toggling on offline CPUs Frederic Weisbecker
2021-01-28 19:52 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up Frederic Weisbecker
[not found] ` <20210128191228.GQ2743@paulmck-ThinkPad-P72>
2021-01-28 21:34 ` Frederic Weisbecker
2021-01-28 21:45 ` Paul E. McKenney [this message]
2021-01-29 0:26 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 05/16] rcu/nocb: Disable bypass when CPU isn't completely offloaded Frederic Weisbecker
2021-01-28 21:31 ` Paul E. McKenney
2021-01-28 22:25 ` Frederic Weisbecker
2021-01-29 0:19 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 06/16] rcu/nocb: Avoid confusing double write of rdp->nocb_cb_sleep Frederic Weisbecker
2021-01-28 21:42 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 07/16] rcu/nocb: Rename nocb_gp_update_state to nocb_gp_update_state_deoffloading Frederic Weisbecker
2021-01-29 0:49 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 08/16] rcu/nocb: Move trace_rcu_nocb_wake() calls outside nocb_lock when possible Frederic Weisbecker
2021-01-29 0:51 ` Paul E. McKenney
2021-01-28 17:12 ` [PATCH 09/16] rcu/nocb: Merge nocb_timer to the rdp leader Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 10/16] rcu/nocb: Directly call __wake_nocb_gp() from bypass timer Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 11/16] rcu/nocb: Allow de-offloading rdp leader Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 12/16] rcu/nocb: Cancel nocb_timer upon nocb_gp wakeup Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 13/16] rcu/nocb: Delete bypass_timer " Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 14/16] rcu/nocb: Only cancel nocb timer if not polling Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 15/16] rcu/nocb: Prepare for finegrained deferred wakeup Frederic Weisbecker
2021-01-28 17:12 ` [PATCH 16/16] rcu/nocb: Unify timers Frederic Weisbecker
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