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

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Message-ID: <cb977466-ad2d-48af-b388-b3c55a5169e2@arm.com>
Date: Fri, 1 Aug 2025 15:54:34 +0100
From: Christian Loehle <christian.loehle@....com>
To: Andrea Righi <arighi@...dia.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 8/1/25 15:51, Andrea Righi wrote:
> 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?

Makes sense to me!

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

Ah, thanks! I'll rebase!