Re: [PATCH] sched_ext: Fix missing unlock in consume_dispatch_q()

[Tejun Heo]

Hello,

On Tue, Oct 15, 2024 at 10:29:17PM +0800, Zhang Qiao wrote:
> When the function consume_dispatch_q() returns true, the dsq lock may
> remains held and is not unlocked.
> 
> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
> ---
>  kernel/sched/ext.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 410a4df8a121..4d80aa3de00e 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -2377,7 +2377,8 @@ static inline bool consume_remote_task(struct rq *this_rq, struct task_struct *p
>  static bool consume_dispatch_q(struct rq *rq, struct scx_dispatch_q *dsq)
>  {
>  	struct task_struct *p;
> -retry:
> +	bool ret = false;
> +
>  	/*
>  	 * The caller can't expect to successfully consume a task if the task's
>  	 * addition to @dsq isn't guaranteed to be visible somehow. Test
> @@ -2394,19 +2395,20 @@ static bool consume_dispatch_q(struct rq *rq, struct scx_dispatch_q *dsq)
>  		if (rq == task_rq) {
>  			task_unlink_from_dsq(p, dsq);
>  			move_local_task_to_local_dsq(p, 0, dsq, rq);
> -			raw_spin_unlock(&dsq->lock);
> -			return true;
> +			ret = true;
> +			break;
>  		}
>  
>  		if (task_can_run_on_remote_rq(p, rq, false)) {
> -			if (likely(consume_remote_task(rq, p, dsq, task_rq)))
> -				return true;
> -			goto retry;
> +			if (likely(consume_remote_task(rq, p, dsq, task_rq))) {
> +				ret = true;
> +				break;
> +			}
>  		}
>  	}
>  
>  	raw_spin_unlock(&dsq->lock);
> -	return false;
> +	return ret;

Has this change been tested at all? There's quite a bit of lock dancing
happening in the remote consumption path because the task needs to get
hot-migrated to the local CPU - consume_remote_task() calls
unlink_dsq_and_lock_src_rq() which drops the DSQ lock among other things.

Thanks.

-- 
tejun