This is a note to let you know that I've just added the patch titled
rcu/nocb: Fix rcuog wake-up from offline softirq
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
rcu-nocb-fix-rcuog-wake-up-from-offline-softirq.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 44261d09fa8413d47477d4518c0ee84dacb589a1
Author: Frederic Weisbecker <frederic@xxxxxxxxxx>
Date: Thu Oct 10 18:36:09 2024 +0200
rcu/nocb: Fix rcuog wake-up from offline softirq
[ 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@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>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 7a0bce3f784df..8993b2322be2b 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -568,13 +568,19 @@ 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"));
