sched_ext: Introduce scx_bpf_locked_rq()

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_locked_rq() as returning NULL as
scx_bpf_cpu_rq() should've been too.

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

2 files changed, 24 insertions, 0 deletions

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 520f20ffb7bf..a319ea5bb25a 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6358,6 +6358,28 @@ __bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu)
 }
 
 /**
+ * scx_bpf_locked_rq - 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_locked_rq(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
  *
@@ -6521,6 +6543,7 @@ 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_locked_rq, 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 86abdb3c3142..e2a57b745862 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -103,6 +103,7 @@ 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_locked_rq(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;