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                       | 23 +++++++++++++++++++++++
 tools/sched_ext/include/scx/common.bpf.h |  1 +
 2 files changed, 24 insertions(+)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 4ae32ef179dd..9fcc310d85d5 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7430,6 +7430,28 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
 	return cpu_rq(cpu);
 }
 
+/**
+ * scx_bpf_cpu_rq_locked - Return the rq currently locked by SCX
+ *
+ * Returns the rq if a rq lock is currently held by SCX.
+ * Otherwise emits an error and returns NULL.
+ */
+__bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(void)
+{
+	struct rq *rq;
+
+	preempt_disable();
+	rq = scx_locked_rq();
+	if (!rq) {
+		preempt_enable();
+		scx_kf_error("accessing rq without holding rq lock");
+		return NULL;
+	}
+	preempt_enable();
+
+	return rq;
+}
+
 /**
  * scx_bpf_task_cgroup - Return the sched cgroup of a task
  * @p: task of interest
@@ -7594,6 +7616,7 @@ 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..f5be06c93359 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(void) __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

On Mon, Sep 01, 2025 at 02:26:02PM +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.
> 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                       | 23 +++++++++++++++++++++++
>  tools/sched_ext/include/scx/common.bpf.h |  1 +
>  2 files changed, 24 insertions(+)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 4ae32ef179dd..9fcc310d85d5 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -7430,6 +7430,28 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
>  	return cpu_rq(cpu);
>  }
>  
> +/**
> + * scx_bpf_cpu_rq_locked - Return the rq currently locked by SCX
> + *
> + * Returns the rq if a rq lock is currently held by SCX.
> + * Otherwise emits an error and returns NULL.
> + */
> +__bpf_kfunc struct rq *scx_bpf_cpu_rq_locked(void)

How about naming it scx_bpf_locked_rq()? That reads a lot easier to me and
given that it doesn't take @cpu anymore, the _cpu_ part of the name isn't
necessary.

Thanks.

-- 
tejun