Re: [PATCH] rust: i2c: avoid locking when calling I2cAdapter::inc_ref

[Igor Korotin]

Hello Nicolás

Sorry for the delay.

Yes, worth fixing regardless of real users -- inc_ref taking a lock it
doesn't need is a real bug, not speculative work.

Instead of adding a separate Rust-only wrapper, please consider splitting
i2c_get_adapter itself: pull the increment into a helper, export it, and
call it directly from inc_ref -- no lock, no lookup:

+bool __i2c_adapter_get(struct i2c_adapter *adapter)
+{
+	if (try_module_get(adapter->owner)) {
+		get_device(&adapter->dev);
+		return true;
+	}
+	return false;
+}
+EXPORT_SYMBOL(__i2c_adapter_get);
+
 struct i2c_adapter *i2c_get_adapter(int nr)
 {
 	struct i2c_adapter *adapter;

 	mutex_lock(&core_lock);
 	adapter = idr_find(&i2c_adapter_idr, nr);
-	if (!adapter)
-		goto exit;
-
-	if (try_module_get(adapter->owner))
-		get_device(&adapter->dev);
-	else
+	if (adapter && !__i2c_adapter_get(adapter))
 		adapter = NULL;
-
- exit:
 	mutex_unlock(&core_lock);
 	return adapter;
 }
 EXPORT_SYMBOL(i2c_get_adapter);

Heads up for v2: Trevor Chan has a patch changing AlwaysRefCounted::inc_ref
to an associated function (fn inc_ref(obj: &Self)). Not merged yet --
rebase onto it if it lands first.

Cheers
Igor