linux-kernel - [PATCH 2/4] sched_ext: idle: Validate locking correctness in scx_bpf_select_cpu

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Message-ID: <20250515191716.327518-3-arighi@nvidia.com>
Date: Thu, 15 May 2025 21:11:43 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH 2/4] sched_ext: idle: Validate locking correctness in scx_bpf_select_cpu_and()

Validate locking correctness when accessing p->nr_cpus_allowed and
p->cpus_ptr inside scx_bpf_select_cpu_and(): if the rq lock is held,
access is safe; otherwise, require that p->pi_lock is held.

This allows to catch potential unsafe calls to scx_bpf_select_cpu_and().

Signed-off-by: Andrea Righi <arighi@...dia.com>
---
 kernel/sched/ext_idle.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index f0ebf8b5b908e..716863f1f8cee 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -935,6 +935,7 @@ __bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct *p, s32 prev_cpu,
 __bpf_kfunc s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
 				       const struct cpumask *cpus_allowed, u64 flags)
 {
+	struct rq *rq;
 	s32 cpu;
 
 	if (!kf_cpu_valid(prev_cpu, NULL))
@@ -946,6 +947,15 @@ __bpf_kfunc s32 scx_bpf_select_cpu_and(struct task_struct *p, s32 prev_cpu, u64
 	if (!scx_kf_allowed(SCX_KF_SELECT_CPU | SCX_KF_ENQUEUE))
 		return -EPERM;
 
+	/*
+	 * Validate locking correctness to access p->cpus_ptr and
+	 * p->nr_cpus_allowed: if we're holding an rq lock, we're safe;
+	 * otherwise, assert that p->pi_lock is held.
+	 */
+	rq = scx_locked_rq();
+	if (!rq)
+		lockdep_assert_held(&p->pi_lock);
+
 #ifdef CONFIG_SMP
 	/*
 	 * This may also be called from ops.enqueue(), so we need to handle
-- 
2.49.0