[PATCH mptcp-next v2 5/5] Squash to "bpf: Add mptcp packet scheduler struct_ops"

[Gang Yan <gang.yan@linux.dev>]

From: Gang Yan <yangang@kylinos.cn>

The pernet sysctl patches in this series swap the scheduler pointer
with xchg() without taking any module reference: this is fine while
ops can only be registered from built-in code.

This commit is what makes BPF schedulers registerable, i.e. the first
one whose ops can be unloaded while a pernet still stores them. Take
the reference accordingly:

  - mptcp_pernet_set_defaults() pins &mptcp_sched_default, mirroring
    tcp_sk_init() pinning &tcp_reno;
  - mptcp_set_scheduler() takes a reference on the new ops and
    releases the one held on the ops it replaces, mirroring
    tcp_set_default_congestion_control();
  - mptcp_net_exit() releases the last reference, mirroring
    tcp_sk_exit().

Assisted-by: Claude:GLM5.2
Co-developed-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Tao Cui <cuitao@kylinos.cn>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
---
 net/mptcp/ctrl.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
index 32268aa26097..309bbb2a4eb0 100644
--- a/net/mptcp/ctrl.c
+++ b/net/mptcp/ctrl.c
@@ -123,7 +123,8 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 	pernet->stale_loss_cnt = 4;
 	pernet->pm_type = MPTCP_PM_TYPE_KERNEL;
 
-	RCU_INIT_POINTER(pernet->scheduler, &mptcp_sched_default);
+	if (bpf_try_module_get(&mptcp_sched_default, mptcp_sched_default.owner))
+		RCU_INIT_POINTER(pernet->scheduler, &mptcp_sched_default);
 
 	if (bpf_try_module_get(&mptcp_pm_kernel, mptcp_pm_kernel.owner))
 		RCU_INIT_POINTER(pernet->path_manager, &mptcp_pm_kernel);
@@ -134,15 +135,22 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
 #ifdef CONFIG_SYSCTL
 static int mptcp_set_scheduler(struct mptcp_pernet *pernet, const char *name)
 {
-	struct mptcp_sched_ops *sched;
+	struct mptcp_sched_ops *sched, *prev;
 	int ret = 0;
 
 	rcu_read_lock();
 	sched = mptcp_sched_find(name);
-	if (sched)
-		xchg(&pernet->scheduler, sched);
-	else
+	if (sched) {
+		if (bpf_try_module_get(sched, sched->owner)) {
+			prev = xchg(&pernet->scheduler, sched);
+			if (prev)
+				bpf_module_put(prev, prev->owner);
+		} else {
+			ret = -EBUSY;
+		}
+	} else {
 		ret = -ENOENT;
+	}
 	rcu_read_unlock();
 
 	return ret;
@@ -603,10 +611,15 @@ static int __net_init mptcp_net_init(struct net *net)
 static void __net_exit mptcp_net_exit(struct net *net)
 {
 	struct mptcp_pernet *pernet = mptcp_get_pernet(net);
+	struct mptcp_sched_ops *sched;
 	struct mptcp_pm_ops *pm;
 
 	mptcp_pernet_del_table(pernet);
 
+	sched = rcu_dereference_protected(pernet->scheduler, true);
+	if (sched)
+		bpf_module_put(sched, sched->owner);
+
 	pm = rcu_dereference_protected(pernet->path_manager, true);
 	if (pm)
 		bpf_module_put(pm, pm->owner);
-- 
2.43.0