Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 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 com.android.server; |
| 18 | |
| 19 | import android.util.ArrayMap; |
| 20 | import android.util.ArraySet; |
| 21 | import android.util.Slog; |
| 22 | |
| 23 | import java.io.FileDescriptor; |
| 24 | import java.io.PrintWriter; |
| 25 | |
| 26 | /** |
| 27 | * LockGuard is a mechanism to help detect lock inversions inside the system |
| 28 | * server. It works by requiring each lock acquisition site to follow this |
| 29 | * pattern: |
| 30 | * |
| 31 | * <pre> |
| 32 | * synchronized (LockGuard.guard(lock)) { |
| 33 | * } |
| 34 | * </pre> |
| 35 | * |
| 36 | * <pre> |
| 37 | * $ find services/ -name "*.java" -exec sed -i -r \ |
| 38 | * 's/synchronized.?\((.+?)\)/synchronized \(com.android.server.LockGuard.guard\(\1\)\)/' {} \; |
| 39 | * </pre> |
| 40 | * |
| 41 | * The {@link #guard(Object)} method internally verifies that all locking is |
| 42 | * done in a consistent order, and will log if any inversion is detected. For |
| 43 | * example, if the calling thread is trying to acquire the |
| 44 | * {@code ActivityManager} lock while holding the {@code PackageManager} lock, |
| 45 | * it will yell. |
| 46 | * <p> |
| 47 | * This class requires no prior knowledge of locks or their ordering; it derives |
| 48 | * all of this data at runtime. However, this means the overhead is |
| 49 | * <em>substantial</em> and it should not be enabled by default. For example, |
| 50 | * here are some benchmarked timings: |
| 51 | * <ul> |
| 52 | * <li>An unguarded synchronized block takes 40ns. |
| 53 | * <li>A guarded synchronized block takes 50ns when disabled. |
| 54 | * <li>A guarded synchronized block takes 460ns per lock checked when enabled. |
| 55 | * </ul> |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 56 | * <p> |
| 57 | * This class also supports a second simpler mode of operation where well-known |
| 58 | * locks are explicitly registered and checked via indexes. |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 59 | */ |
| 60 | public class LockGuard { |
| 61 | private static final String TAG = "LockGuard"; |
| 62 | |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 63 | public static final boolean ENABLED = false; |
| 64 | |
| 65 | /** |
| 66 | * Well-known locks ordered by fixed index. Locks with a specific index |
| 67 | * should never be acquired while holding a lock of a lower index. |
| 68 | */ |
| 69 | public static final int INDEX_APP_OPS = 0; |
| 70 | public static final int INDEX_POWER = 1; |
| 71 | public static final int INDEX_USER = 2; |
| 72 | public static final int INDEX_PACKAGES = 3; |
| 73 | public static final int INDEX_STORAGE = 4; |
| 74 | public static final int INDEX_WINDOW = 5; |
| 75 | public static final int INDEX_ACTIVITY = 6; |
| 76 | |
| 77 | private static Object[] sKnownFixed = new Object[INDEX_ACTIVITY + 1]; |
| 78 | |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 79 | private static ArrayMap<Object, LockInfo> sKnown = new ArrayMap<>(0, true); |
| 80 | |
| 81 | private static class LockInfo { |
| 82 | /** Friendly label to describe this lock */ |
| 83 | public String label; |
| 84 | |
| 85 | /** Child locks that can be acquired while this lock is already held */ |
| 86 | public ArraySet<Object> children = new ArraySet<>(0, true); |
| 87 | } |
| 88 | |
| 89 | private static LockInfo findOrCreateLockInfo(Object lock) { |
| 90 | LockInfo info = sKnown.get(lock); |
| 91 | if (info == null) { |
| 92 | info = new LockInfo(); |
| 93 | info.label = "0x" + Integer.toHexString(System.identityHashCode(lock)) + " [" |
| 94 | + new Throwable().getStackTrace()[2].toString() + "]"; |
| 95 | sKnown.put(lock, info); |
| 96 | } |
| 97 | return info; |
| 98 | } |
| 99 | |
| 100 | /** |
| 101 | * Check if the calling thread is holding any locks in an inverted order. |
| 102 | * |
| 103 | * @param lock The lock the calling thread is attempting to acquire. |
| 104 | */ |
| 105 | public static Object guard(Object lock) { |
| 106 | // If we already hold this lock, ignore |
| 107 | if (lock == null || Thread.holdsLock(lock)) return lock; |
| 108 | |
| 109 | // Check to see if we're already holding any child locks |
| 110 | boolean triggered = false; |
| 111 | final LockInfo info = findOrCreateLockInfo(lock); |
| 112 | for (int i = 0; i < info.children.size(); i++) { |
| 113 | final Object child = info.children.valueAt(i); |
| 114 | if (child == null) continue; |
| 115 | |
| 116 | if (Thread.holdsLock(child)) { |
| 117 | Slog.w(TAG, "Calling thread " + Thread.currentThread().getName() + " is holding " |
| 118 | + lockToString(child) + " while trying to acquire " |
| 119 | + lockToString(lock), new Throwable()); |
| 120 | triggered = true; |
| 121 | } |
| 122 | } |
| 123 | |
| 124 | if (!triggered) { |
| 125 | // If no trouble found above, record this lock as being a valid |
| 126 | // child of all locks currently being held |
| 127 | for (int i = 0; i < sKnown.size(); i++) { |
| 128 | final Object test = sKnown.keyAt(i); |
| 129 | if (test == null || test == lock) continue; |
| 130 | |
| 131 | if (Thread.holdsLock(test)) { |
| 132 | sKnown.valueAt(i).children.add(lock); |
| 133 | } |
| 134 | } |
| 135 | } |
| 136 | |
| 137 | return lock; |
| 138 | } |
| 139 | |
| 140 | /** |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 141 | * Yell if any lower-level locks are being held by the calling thread that |
| 142 | * is about to acquire the given lock. |
| 143 | */ |
| 144 | public static void guard(int index) { |
| 145 | for (int i = 0; i < index; i++) { |
| 146 | final Object lock = sKnownFixed[i]; |
| 147 | if (lock != null && Thread.holdsLock(lock)) { |
| 148 | Slog.w(TAG, "Calling thread " + Thread.currentThread().getName() + " is holding " |
| 149 | + lockToString(i) + " while trying to acquire " |
| 150 | + lockToString(index), new Throwable()); |
| 151 | } |
| 152 | } |
| 153 | } |
| 154 | |
| 155 | /** |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 156 | * Report the given lock with a well-known label. |
| 157 | */ |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 158 | public static Object installLock(Object lock, String label) { |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 159 | final LockInfo info = findOrCreateLockInfo(lock); |
| 160 | info.label = label; |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 161 | return lock; |
| 162 | } |
| 163 | |
| 164 | /** |
| 165 | * Report the given lock with a well-known index. |
| 166 | */ |
| 167 | public static Object installLock(Object lock, int index) { |
| 168 | sKnownFixed[index] = lock; |
| 169 | return lock; |
| 170 | } |
| 171 | |
| 172 | public static Object installNewLock(int index) { |
| 173 | final Object lock = new Object(); |
| 174 | installLock(lock, index); |
| 175 | return lock; |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 176 | } |
| 177 | |
| 178 | private static String lockToString(Object lock) { |
| 179 | final LockInfo info = sKnown.get(lock); |
| 180 | if (info != null) { |
| 181 | return info.label; |
| 182 | } else { |
| 183 | return "0x" + Integer.toHexString(System.identityHashCode(lock)); |
| 184 | } |
| 185 | } |
| 186 | |
Jeff Sharkey | 5f3e934 | 2017-03-13 14:53:11 -0600 | [diff] [blame] | 187 | private static String lockToString(int index) { |
| 188 | switch (index) { |
| 189 | case INDEX_APP_OPS: return "APP_OPS"; |
| 190 | case INDEX_POWER: return "POWER"; |
| 191 | case INDEX_USER: return "USER"; |
| 192 | case INDEX_PACKAGES: return "PACKAGES"; |
| 193 | case INDEX_STORAGE: return "STORAGE"; |
| 194 | case INDEX_WINDOW: return "WINDOW"; |
| 195 | case INDEX_ACTIVITY: return "ACTIVITY"; |
| 196 | default: return Integer.toString(index); |
| 197 | } |
| 198 | } |
| 199 | |
Jeff Sharkey | 3d1cb6a | 2016-02-27 21:10:34 -0700 | [diff] [blame] | 200 | public static void dump(FileDescriptor fd, PrintWriter pw, String[] args) { |
| 201 | for (int i = 0; i < sKnown.size(); i++) { |
| 202 | final Object lock = sKnown.keyAt(i); |
| 203 | final LockInfo info = sKnown.valueAt(i); |
| 204 | pw.println("Lock " + lockToString(lock) + ":"); |
| 205 | for (int j = 0; j < info.children.size(); j++) { |
| 206 | pw.println(" Child " + lockToString(info.children.valueAt(j))); |
| 207 | } |
| 208 | pw.println(); |
| 209 | } |
| 210 | } |
| 211 | } |