Re: [PATCH 04/16] rcu/nocb: Only (re-)initialize segcblist when needed on CPU up

[Paul E. McKenney]

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@xxxxxxxxxx>
> > > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > > Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > 
> > 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