Re: [PATCH] sched: Fix race between yield_to() and try_to_wake_up()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 31, 2024 at 01:50:20PM +0800, Tianchen Ding wrote:
> We met a SCHED_WARN in set_next_buddy():
>   __warn_printk
>   set_next_buddy
>   yield_to_task_fair
>   yield_to
>   kvm_vcpu_yield_to [kvm]
>   ...
> 
> After a short dig, we found the rq_lock held by yield_to() may not
> be exactly the rq that the target task belongs to. There is a race
> window against try_to_wake_up().
> 
>          CPU0                             target_task
> 
>                                         blocking on CPU1
>    lock rq0 & rq1
>    double check task_rq == p_rq, ok
>                                         woken to CPU2 (lock task_pi & rq2)
>                                         task_rq = rq2
>    yield_to_task_fair (w/o lock rq2)
> 
> In this race window, yield_to() is operating the task w/o the currect
> lock. Fix this by taking task pi_lock first.
> 
> Fixes: d95f41220065 ("sched: Add yield_to(task, preempt) functionality")
> Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
> ---
>  kernel/sched/syscalls.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index ff0e5ab4e37c..943406c4ee86 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -1433,7 +1433,7 @@ int __sched yield_to(struct task_struct *p, bool preempt)
>  	struct rq *rq, *p_rq;
>  	int yielded = 0;
>  
> -	scoped_guard (irqsave) {
> +	scoped_guard (raw_spinlock_irqsave, &p->pi_lock) {
>  		rq = this_rq();

Thanks!