[PATCH 2/2] sched_ext: Fix missing rq lock in scx_bpf_cpuperf_set()

Andrea Righi
2025-04-19T12:36:08Z

scx_bpf_cpuperf_set() can be used to set a performance target level on
any CPU. However, it doesn't correctly acquire the corresponding rq
lock, which may lead to unsafe behavior and trigger the following
warning, due to the lockdep_assert_rq_held() check:

[ 51.713737] WARNING: CPU: 3 PID: 3899 at kernel/sched/sched.h:1512 scx_bpf_cpuperf_set+0x1a0/0x1e0
...
[ 51.713836] Call trace:
[ 51.713837] scx_bpf_cpuperf_set+0x1a0/0x1e0 (P)
[ 51.713839] bpf_prog_62d35beb9301601f_bpfland_init+0x168/0x440
[ 51.713841] bpf__sched_ext_ops_init+0x54/0x8c
[ 51.713843] scx_ops_enable.constprop.0+0x2c0/0x10f0
[ 51.713845] bpf_scx_reg+0x18/0x30
[ 51.713847] bpf_struct_ops_link_create+0x154/0x1b0
[ 51.713849] __sys_bpf+0x1934/0x22a0

Fix by properly acquiring the rq lock when possible or raising an error
if we try to operate on a CPU that is not the one currently locked.

Fixes: d86adb4fc0655 ("sched_ext: Add cpuperf support")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
kernel/sched/ext.c | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index a14a5c3bc38ac..eddd84731c019 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7078,13 +7078,32 @@ __bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf)
}

if (ops_cpu_valid(cpu, NULL)) {
- struct rq *rq = cpu_rq(cpu);
+ struct rq *rq = cpu_rq(cpu), *locked_rq = scx_locked_rq();
+ struct rq_flags rf;
+
+ /*
+ * When called with an rq lock held, restrict the operation
+ * to the corresponding CPU to prevent ABBA deadlocks.
+ */
+ if (locked_rq && rq != locked_rq) {
+ scx_error("Invalid target CPU %d", cpu);
+ return;
+ }
+
+ /*
+ * If no rq lock is held, allow to operate on any CPU by
+ * acquiring the corresponding rq lock.
+ */
+ if (!locked_rq) {
+ rq_lock_irqsave(rq, &rf);
+ update_rq_clock(rq);
+ }

rq->scx.cpuperf_target = perf;
+ cpufreq_update_util(rq, 0);

- rcu_read_lock_sched_notrace();
- cpufreq_update_util(cpu_rq(cpu), 0);
- rcu_read_unlock_sched_notrace();
+ if (!locked_rq)
+ rq_unlock_irqrestore(rq, &rf);
}
}

--
2.49.0