6.11-stable review patch. If anyone has any objections, please let me know.
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From: Frederic Weisbecker frederic@kernel.org
[ Upstream commit f7345ccc62a4b880cf76458db5f320725f28e400 ]
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@intel.com Closes: https://lore.kernel.org/oe-lkp/202409231644.4c55582d-lkp@intel.com Fixes: 9139f93209d1 ("rcu/nocb: Fix RT throttling hrtimer armed from offline CPU") Reviewed-by: "Joel Fernandes (Google)" joel@joelfernandes.org Signed-off-by: Frederic Weisbecker frederic@kernel.org Signed-off-by: Neeraj Upadhyay neeraj.upadhyay@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 2686ba122fa08..3630f712358e4 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -569,13 +569,19 @@ 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"));