1 /* 2 * Copyright (C) 2016 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package dalvik.annotation.optimization; 18 19 import java.lang.annotation.ElementType; 20 import java.lang.annotation.Retention; 21 import java.lang.annotation.RetentionPolicy; 22 import java.lang.annotation.Target; 23 24 /** 25 * An ART runtime built-in optimization for "native" methods to speed up JNI transitions. 26 * 27 * <p> 28 * This has the side-effect of disabling all garbage collections while executing a fast native 29 * method. Use with extreme caution. Any long-running methods must not be marked with 30 * {@code @FastNative} (including usually-fast but generally unbounded methods)!</p> 31 * 32 * <p><b>Deadlock Warning:</b>As a rule of thumb, do not acquire any locks during a fast native 33 * call if they aren't also locally released [before returning to managed code].</p> 34 * 35 * <p> 36 * Say some code does: 37 * 38 * <code> 39 * fast_jni_call_to_grab_a_lock(); 40 * does_some_java_work(); 41 * fast_jni_call_to_release_a_lock(); 42 * </code> 43 * 44 * <p> 45 * This code can lead to deadlocks. Say thread 1 just finishes 46 * {@code fast_jni_call_to_grab_a_lock()} and is in {@code does_some_java_work()}. 47 * GC kicks in and suspends thread 1. Thread 2 now is in {@code fast_jni_call_to_grab_a_lock()} 48 * but is blocked on grabbing the native lock since it's held by thread 1. 49 * Now thread suspension can't finish since thread 2 can't be suspended since it's doing 50 * FastNative JNI. 51 * </p> 52 * 53 * <p> 54 * Normal JNI doesn't have the issue since once it's in native code, 55 * it is considered suspended from java's point of view. 56 * FastNative JNI however doesn't do the state transition done by JNI. 57 * </p> 58 * 59 * <p> 60 * Has no effect when used with non-native methods. 61 * </p> 62 * 63 * @hide 64 */ 65 @libcore.api.CorePlatformApi 66 @Retention(RetentionPolicy.CLASS) // Save memory, don't instantiate as an object at runtime. 67 @Target(ElementType.METHOD) 68 public @interface FastNative {} 69