Most fields in scx_bpf_cpu_rq() assume that its rq_lock is held.
Furthermore they become meaningless without rq lock, too.
Make a safer version of scx_bpf_cpu_rq() that only returns a rq
if we hold rq lock of that rq.

Also mark the new scx_bpf_cpu_rq_locked() as returning NULL.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
---
 kernel/sched/ext.c                       | 22 ++++++++++++++++++++++
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 2 files changed, 23 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7dedc9a16281..14706c36ca83 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7426,6 +7426,27 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
 	return cpu_rq(cpu);
 }
 
+/**
+ * scx_bpf_cpu_rq_locked - Fetch the locked rq of a CPU
+ * @cpu: CPU of the rq
+ */
+__bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(s32 cpu)
+{
+	struct rq *rq;
+
+	if (!kf_cpu_valid(cpu, NULL))
+		return NULL;
+
+	preempt_disable();
+	rq = cpu_rq(cpu);
+	if (rq != scx_locked_rq()) {
+		scx_kf_error("Accessing not locked rq %d", cpu);
+		rq = NULL;
+	}
+	preempt_enable();
+	return rq;
+}
+
 /**
  * scx_bpf_task_cgroup - Return the sched cgroup of a task
  * @p: task of interest
@@ -7590,6 +7611,7 @@ BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
 BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
 BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
+BTF_ID_FLAGS(func, scx_bpf_cpu_rq_locked, KF_RET_NULL)
 #ifdef CONFIG_CGROUP_SCHED
 BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_RCU | KF_ACQUIRE)
 #endif
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index d4e21558e982..7451491347ed 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -91,6 +91,7 @@ s32 scx_bpf_pick_any_cpu(const cpumask_t *cpus_allowed, u64 flags) __ksym;
 bool scx_bpf_task_running(const struct task_struct *p) __ksym;
 s32 scx_bpf_task_cpu(const struct task_struct *p) __ksym;
 struct rq *scx_bpf_cpu_rq(s32 cpu) __ksym;
+struct rq *scx_bpf_cpu_rq_locked(s32 cpu) __ksym;
 struct cgroup *scx_bpf_task_cgroup(struct task_struct *p) __ksym __weak;
 u64 scx_bpf_now(void) __ksym __weak;
 void scx_bpf_events(struct scx_event_stats *events, size_t events__sz) __ksym __weak;
-- 
2.34.1

Hello,

On Mon, Aug 11, 2025 at 10:21:48PM +0100, Christian Loehle wrote:
> +/**
> + * scx_bpf_cpu_rq_locked - Fetch the locked rq of a CPU
> + * @cpu: CPU of the rq
> + */
> +__bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(s32 cpu)
> +{
> +	struct rq *rq;
> +
> +	if (!kf_cpu_valid(cpu, NULL))
> +		return NULL;
> +
> +	preempt_disable();
> +	rq = cpu_rq(cpu);
> +	if (rq != scx_locked_rq()) {
> +		scx_kf_error("Accessing not locked rq %d", cpu);
> +		rq = NULL;
> +	}
> +	preempt_enable();
> +	return rq;
> +}

Do we need @cpu? What do you think about making the function not take any
arguments and just return the locked rq?

Thanks.

-- 
tejun

On 8/12/25 00:38, Tejun Heo wrote:
> Hello,
> 
> On Mon, Aug 11, 2025 at 10:21:48PM +0100, Christian Loehle wrote:
>> +/**
>> + * scx_bpf_cpu_rq_locked - Fetch the locked rq of a CPU
>> + * @cpu: CPU of the rq
>> + */
>> +__bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(s32 cpu)
>> +{
>> +	struct rq *rq;
>> +
>> +	if (!kf_cpu_valid(cpu, NULL))
>> +		return NULL;
>> +
>> +	preempt_disable();
>> +	rq = cpu_rq(cpu);
>> +	if (rq != scx_locked_rq()) {
>> +		scx_kf_error("Accessing not locked rq %d", cpu);
>> +		rq = NULL;
>> +	}
>> +	preempt_enable();
>> +	return rq;
>> +}
> 
> Do we need @cpu? What do you think about making the function not take any
> arguments and just return the locked rq?

Indeed now that this no longer has to be a drop-in replacement.