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

[Andrea Righi <arighi@nvidia.com>]

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 3365b447cbdb8..a175b622716ce 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -7088,13 +7088,32 @@ __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