[PATCH v2 bpf-next 1/3] bpf: eliminate btf_module_mutex as RCU synchronization can be used

[Alan Maguire <alan.maguire@xxxxxxxxxx>]

btf_module_mutex is used when manipulating the BTF module list.
However we will wish to look up this list from BPF program context,
and such contexts can include interrupt state where we cannot sleep
due to a mutex_lock().  RCU usage here conforms quite closely
to the example in the system call auditing example in
Documentation/RCU/listRCU.rst ; and as such we can eliminate
the lock and use list_del_rcu()/call_rcu() on module removal,
and list_add_rcu() for module addition.

Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
---
 kernel/bpf/btf.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 8d6bdb4..333f41c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5758,13 +5758,13 @@ bool btf_id_set_contains(const struct btf_id_set *set, u32 id)
 #ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 struct btf_module {
 	struct list_head list;
+	struct rcu_head	rcu;
 	struct module *module;
 	struct btf *btf;
 	struct bin_attribute *sysfs_attr;
 };
 
 static LIST_HEAD(btf_modules);
-static DEFINE_MUTEX(btf_module_mutex);
 
 static ssize_t
 btf_module_read(struct file *file, struct kobject *kobj,
@@ -5777,10 +5777,21 @@ struct btf_module {
 	return len;
 }
 
+static void btf_module_free(struct rcu_head *rcu)
+{
+	struct btf_module *btf_mod = container_of(rcu, struct btf_module, rcu);
+
+	if (btf_mod->sysfs_attr)
+		sysfs_remove_bin_file(btf_kobj, btf_mod->sysfs_attr);
+	btf_put(btf_mod->btf);
+	kfree(btf_mod->sysfs_attr);
+	kfree(btf_mod);
+}
+
 static int btf_module_notify(struct notifier_block *nb, unsigned long op,
 			     void *module)
 {
-	struct btf_module *btf_mod, *tmp;
+	struct btf_module *btf_mod;
 	struct module *mod = module;
 	struct btf *btf;
 	int err = 0;
@@ -5811,11 +5822,9 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
 			goto out;
 		}
 
-		mutex_lock(&btf_module_mutex);
 		btf_mod->module = module;
 		btf_mod->btf = btf;
-		list_add(&btf_mod->list, &btf_modules);
-		mutex_unlock(&btf_module_mutex);
+		list_add_rcu(&btf_mod->list, &btf_modules);
 
 		if (IS_ENABLED(CONFIG_SYSFS)) {
 			struct bin_attribute *attr;
@@ -5845,20 +5854,14 @@ static int btf_module_notify(struct notifier_block *nb, unsigned long op,
 
 		break;
 	case MODULE_STATE_GOING:
-		mutex_lock(&btf_module_mutex);
-		list_for_each_entry_safe(btf_mod, tmp, &btf_modules, list) {
+		list_for_each_entry(btf_mod, &btf_modules, list) {
 			if (btf_mod->module != module)
 				continue;
 
-			list_del(&btf_mod->list);
-			if (btf_mod->sysfs_attr)
-				sysfs_remove_bin_file(btf_kobj, btf_mod->sysfs_attr);
-			btf_put(btf_mod->btf);
-			kfree(btf_mod->sysfs_attr);
-			kfree(btf_mod);
+			list_del_rcu(&btf_mod->list);
+			call_rcu(&btf_mod->rcu, btf_module_free);
 			break;
 		}
-		mutex_unlock(&btf_module_mutex);
 		break;
 	}
 out:
-- 
1.8.3.1