linux-kernel - Re: [PATCH 3/3] sched_ext: Guarantee rq lock on scx_bpf_cpu

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Message-ID: <aIzUX4d0KRXI4E8O@gpd4>
Date: Fri, 1 Aug 2025 16:51:11 +0200
From: Andrea Righi <arighi@...dia.com>
To: Christian Loehle <christian.loehle@....com>
Cc: linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev, tj@...nel.org,
	void@...ifault.com, mingo@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 3/3] sched_ext: Guarantee rq lock on scx_bpf_cpu_rq()

On Fri, Aug 01, 2025 at 03:17:41PM +0100, Christian Loehle wrote:
> Most fields in scx_bpf_cpu_rq() assume that its rq_lock is held.
> Furthermore they become meaningless without rq lock, too.
> Only return scx_bpf_cpu_rq() when we hold rq lock of that rq.
> 
> All upstream scx schedulers can be converted into the new
> scx_bpf_remote_curr() instead.
> 
> Signed-off-by: Christian Loehle <christian.loehle@....com>
> ---
>  kernel/sched/ext.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 92e66bb0b5f2..627df3088fd0 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7425,7 +7425,7 @@ __bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
>  }
>  
>  /**
> - * scx_bpf_cpu_rq - Fetch the rq of a CPU
> + * scx_bpf_cpu_rq - Fetch the rq of a CPU if its rq lock is currently held
>   * @cpu: CPU of the rq
>   */
>  __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
> @@ -7433,7 +7433,7 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
>  	if (!kf_cpu_valid(cpu, NULL))
>  		return NULL;
>  
> -	return cpu_rq(cpu);
> +	return this_cpu_read(locked_rq) == cpu_rq(cpu) ? cpu_rq(cpu) : NULL;

Maybe we should consider an access to an unlocked rq as invalid and trigger
scx_exit(), similar to what we do with the kf_cpu_valid() check?

Also heads up that locked_rq has been renamed scx_locked_rq_state in 6.17.

>  }
>  
>  /**
> -- 
> 2.34.1
> 

Thanks,
-Andrea