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
>
> 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
>
>
> 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
> Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-...@intel.com
> Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline
> CPU")
> Reviewed-by: Joel Fernandes (Google)
> Signed-off-by: Frederic Weisbecker
Reviewed-by: Paul E. McKenney
> ---
> 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
>
