Re: [syzbot] [kernel?] KCSAN: assert: race in dequeue_entities

[Peter Zijlstra]

On Tue, Oct 22, 2024 at 03:40:52PM +0200, Marco Elver wrote:

> Which gives us:
> 
>  | ==================================================================
>  | BUG: KCSAN: assert: race in dequeue_entities / ttwu_do_activate
>  | 
>  | write (marked) to 0xffff9e100329c628 of 4 bytes by interrupt on cpu 0:
>  |  activate_task kernel/sched/core.c:2064 [inline]
> 
> This is this one:
> 
> 	void activate_task(struct rq *rq, struct task_struct *p, int flags)
> 	{
> 		if (task_on_rq_migrating(p))
> 			flags |= ENQUEUE_MIGRATED;
> 		if (flags & ENQUEUE_MIGRATED)
> 			sched_mm_cid_migrate_to(rq, p);
> 
> 		enqueue_task(rq, p, flags);
> 
> 		WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
> 		ASSERT_EXCLUSIVE_WRITER(p->on_rq);
> 	}
> 
>  |  ttwu_do_activate+0x153/0x3e0 kernel/sched/core.c:3671
>  |  ttwu_queue kernel/sched/core.c:3944 [inline]
>  |  try_to_wake_up+0x60f/0xaf0 kernel/sched/core.c:4270

>  | assert no writes to 0xffff9e100329c628 of 4 bytes by task 10571 on cpu 3:
>  |  __block_task kernel/sched/sched.h:2770 [inline]
> 
> And that's:
> 
> 	static inline void __block_task(struct rq *rq, struct task_struct *p)
> 	{
> 		WRITE_ONCE(p->on_rq, 0);
> 		ASSERT_EXCLUSIVE_WRITER(p->on_rq);
> 		if (p->sched_contributes_to_load)
> 			rq->nr_uninterruptible++;
> 
>  |  dequeue_entities+0xd83/0xe70 kernel/sched/fair.c:7177
>  |  pick_next_entity kernel/sched/fair.c:5627 [inline]
>  |  pick_task_fair kernel/sched/fair.c:8856 [inline]
>  |  pick_next_task_fair+0xaf/0x710 kernel/sched/fair.c:8876
>  |  __pick_next_task kernel/sched/core.c:5955 [inline]
>  |  pick_next_task kernel/sched/core.c:6477 [inline]
>  |  __schedule+0x47a/0x1130 kernel/sched/core.c:6629
>  |  __schedule_loop kernel/sched/core.c:6752 [inline]
>  |  schedule+0x7b/0x130 kernel/sched/core.c:6767


So KCSAn is trying to tell me these two paths run concurrently on the
same 'p' ?!? That would be a horrible bug -- both these call chains
should be holding rq->__lock (for task_rq(p)).