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
