On Thu, Oct 10, 2024 at 06:36:09PM +0200, Frederic Weisbecker wrote: > After a CPU has set itself offline and before it eventually calls > rcutree_report_cpu_dead(), there are still opportunities for callbacks > to be enqueued, for example from a softirq. When that happens on NOCB, > the rcuog wake-up is deferred through an IPI to an online CPU in order > not to call into the scheduler and risk arming the RT-bandwidth after > hrtimers have been migrated out and disabled. > > But performing a synchronized IPI from a softirq is buggy as reported in > the following scenario: > > WARNING: CPU: 1 PID: 26 at kernel/smp.c:633 smp_call_function_single > Modules linked in: rcutorture torture > CPU: 1 UID: 0 PID: 26 Comm: migration/1 Not tainted 6.11.0-rc1-00012-g9139f93209d1 #1 > Stopper: multi_cpu_stop+0x0/0x320 <- __stop_cpus+0xd0/0x120 > RIP: 0010:smp_call_function_single > <IRQ> > swake_up_one_online > __call_rcu_nocb_wake > __call_rcu_common > ? rcu_torture_one_read > call_timer_fn > __run_timers > run_timer_softirq > handle_softirqs > irq_exit_rcu > ? tick_handle_periodic > sysvec_apic_timer_interrupt > </IRQ> > > Fix this with forcing deferred rcuog wake up through the NOCB timer when > the CPU is offline. The actual wake up will happen from > rcutree_report_cpu_dead(). > > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx> > Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@xxxxxxxxx > Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU") > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > --- > kernel/rcu/tree_nocb.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index 97b99cd06923..16865475120b 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -554,13 +554,19 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, > rcu_nocb_unlock(rdp); > wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, > TPS("WakeLazy")); > - } else if (!irqs_disabled_flags(flags)) { > + } else if (!irqs_disabled_flags(flags) && cpu_online(rdp->cpu)) { > /* ... if queue was empty ... */ > rcu_nocb_unlock(rdp); > wake_nocb_gp(rdp, false); > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, > TPS("WakeEmpty")); > } else { > + /* > + * Don't do the wake-up upfront on fragile paths. > + * Also offline CPUs can't call swake_up_one_online() from > + * (soft-)IRQs. Rely on the final deferred wake-up from > + * rcutree_report_cpu_dead() > + */ > rcu_nocb_unlock(rdp); > wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE, > TPS("WakeEmptyIsDeferred")); > -- > 2.46.0 >
