rcu/nocb: Fix rcuog wake-up from offline softirq

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 <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Fixes: 9139f93 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU")
Reviewed-by: "Joel Fernandes (Google)" <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

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"));