6.15-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
[ Upstream commit 009836b4fa52f92cba33618e773b1094affa8cd2 ]
On Mon, Jun 02, 2025 at 03:22:13PM +0800, Kuyo Chang wrote:
So, the potential race scenario is:
CPU0 CPU1 // doing migrate_swap(cpu0/cpu1) stop_two_cpus() ... // doing _cpu_down() sched_cpu_deactivate() set_cpu_active(cpu, false); balance_push_set(cpu, true); cpu_stop_queue_two_works __cpu_stop_queue_work(stopper1,...); __cpu_stop_queue_work(stopper2,..); stop_cpus_in_progress -> true preempt_enable(); ... 1st balance_push stop_one_cpu_nowait cpu_stop_queue_work __cpu_stop_queue_work list_add_tail -> 1st add push_work wake_up_q(&wakeq); -> "wakeq is empty. This implies that the stopper is at wakeq@migrate_swap." preempt_disable wake_up_q(&wakeq); wake_up_process // wakeup migrate/0 try_to_wake_up ttwu_queue ttwu_queue_cond ->meet below case if (cpu == smp_processor_id()) return false; ttwu_do_activate //migrate/0 wakeup done wake_up_process // wakeup migrate/1 try_to_wake_up ttwu_queue ttwu_queue_cond ttwu_queue_wakelist __ttwu_queue_wakelist __smp_call_single_queue preempt_enable();
2nd balance_push stop_one_cpu_nowait cpu_stop_queue_work __cpu_stop_queue_work list_add_tail -> 2nd add push_work, so the double list add is detected ... ... cpu1 get ipi, do sched_ttwu_pending, wakeup migrate/1
So this balance_push() is part of schedule(), and schedule() is supposed to switch to stopper task, but because of this race condition, stopper task is stuck in WAKING state and not actually visible to be picked.
Therefore CPU1 can do another schedule() and end up doing another balance_push() even though the last one hasn't been done yet.
This is a confluence of fail, where both wake_q and ttwu_wakelist can cause crucial wakeups to be delayed, resulting in the malfunction of balance_push.
Since there is only a single stopper thread to be woken, the wake_q doesn't really add anything here, and can be removed in favour of direct wakeups of the stopper thread.
Then add a clause to ttwu_queue_cond() to ensure the stopper threads are never queued / delayed.
Of all 3 moving parts, the last addition was the balance_push() machinery, so pick that as the point the bug was introduced.
Fixes: 2558aacff858 ("sched/hotplug: Ensure only per-cpu kthreads run during hotplug") Reported-by: Kuyo Chang kuyo.chang@mediatek.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Tested-by: Kuyo Chang kuyo.chang@mediatek.com Link: https://lkml.kernel.org/r/20250605100009.GO39944@noisy.programming.kicks-ass... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/core.c | 5 +++++ kernel/stop_machine.c | 20 ++++++++++---------- 2 files changed, 15 insertions(+), 10 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 39fac649aa142..566fbf0c1b0b2 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3935,6 +3935,11 @@ static inline bool ttwu_queue_cond(struct task_struct *p, int cpu) if (!scx_allow_ttwu_queue(p)) return false;
+#ifdef CONFIG_SMP + if (p->sched_class == &stop_sched_class) + return false; +#endif + /* * Do not complicate things with the async wake_list while the CPU is * in hotplug state. diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c index 5d2d0562115b3..3fe6b0c99f3d8 100644 --- a/kernel/stop_machine.c +++ b/kernel/stop_machine.c @@ -82,18 +82,15 @@ static void cpu_stop_signal_done(struct cpu_stop_done *done) }
static void __cpu_stop_queue_work(struct cpu_stopper *stopper, - struct cpu_stop_work *work, - struct wake_q_head *wakeq) + struct cpu_stop_work *work) { list_add_tail(&work->list, &stopper->works); - wake_q_add(wakeq, stopper->thread); }
/* queue @work to @stopper. if offline, @work is completed immediately */ static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work) { struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu); - DEFINE_WAKE_Q(wakeq); unsigned long flags; bool enabled;
@@ -101,12 +98,13 @@ static bool cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work) raw_spin_lock_irqsave(&stopper->lock, flags); enabled = stopper->enabled; if (enabled) - __cpu_stop_queue_work(stopper, work, &wakeq); + __cpu_stop_queue_work(stopper, work); else if (work->done) cpu_stop_signal_done(work->done); raw_spin_unlock_irqrestore(&stopper->lock, flags);
- wake_up_q(&wakeq); + if (enabled) + wake_up_process(stopper->thread); preempt_enable();
return enabled; @@ -264,7 +262,6 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, { struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1); struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2); - DEFINE_WAKE_Q(wakeq); int err;
retry: @@ -300,8 +297,8 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, }
err = 0; - __cpu_stop_queue_work(stopper1, work1, &wakeq); - __cpu_stop_queue_work(stopper2, work2, &wakeq); + __cpu_stop_queue_work(stopper1, work1); + __cpu_stop_queue_work(stopper2, work2);
unlock: raw_spin_unlock(&stopper2->lock); @@ -316,7 +313,10 @@ static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1, goto retry; }
- wake_up_q(&wakeq); + if (!err) { + wake_up_process(stopper1->thread); + wake_up_process(stopper2->thread); + } preempt_enable();
return err;