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 <frede...@kernel.org>
> > > Cc: Josh Triplett <j...@joshtriplett.org>
> > > Cc: Lai Jiangshan <jiangshan...@gmail.com>
> > > Cc: Joel Fernandes <j...@joelfernandes.org>
> > > Cc: Neeraj Upadhyay <neer...@codeaurora.org>
> > > Cc: Boqun Feng <boqun.f...@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
