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> |
| 56 | */ |
| 57 | public class LockGuard { |
| 58 | private static final String TAG = "LockGuard"; |
| 59 | |
| 60 | private static ArrayMap<Object, LockInfo> sKnown = new ArrayMap<>(0, true); |
| 61 | |
| 62 | private static class LockInfo { |
| 63 | /** Friendly label to describe this lock */ |
| 64 | public String label; |
| 65 | |
| 66 | /** Child locks that can be acquired while this lock is already held */ |
| 67 | public ArraySet<Object> children = new ArraySet<>(0, true); |
| 68 | } |
| 69 | |
| 70 | private static LockInfo findOrCreateLockInfo(Object lock) { |
| 71 | LockInfo info = sKnown.get(lock); |
| 72 | if (info == null) { |
| 73 | info = new LockInfo(); |
| 74 | info.label = "0x" + Integer.toHexString(System.identityHashCode(lock)) + " [" |
| 75 | + new Throwable().getStackTrace()[2].toString() + "]"; |
| 76 | sKnown.put(lock, info); |
| 77 | } |
| 78 | return info; |
| 79 | } |
| 80 | |
| 81 | /** |
| 82 | * Check if the calling thread is holding any locks in an inverted order. |
| 83 | * |
| 84 | * @param lock The lock the calling thread is attempting to acquire. |
| 85 | */ |
| 86 | public static Object guard(Object lock) { |
| 87 | // If we already hold this lock, ignore |
| 88 | if (lock == null || Thread.holdsLock(lock)) return lock; |
| 89 | |
| 90 | // Check to see if we're already holding any child locks |
| 91 | boolean triggered = false; |
| 92 | final LockInfo info = findOrCreateLockInfo(lock); |
| 93 | for (int i = 0; i < info.children.size(); i++) { |
| 94 | final Object child = info.children.valueAt(i); |
| 95 | if (child == null) continue; |
| 96 | |
| 97 | if (Thread.holdsLock(child)) { |
| 98 | Slog.w(TAG, "Calling thread " + Thread.currentThread().getName() + " is holding " |
| 99 | + lockToString(child) + " while trying to acquire " |
| 100 | + lockToString(lock), new Throwable()); |
| 101 | triggered = true; |
| 102 | } |
| 103 | } |
| 104 | |
| 105 | if (!triggered) { |
| 106 | // If no trouble found above, record this lock as being a valid |
| 107 | // child of all locks currently being held |
| 108 | for (int i = 0; i < sKnown.size(); i++) { |
| 109 | final Object test = sKnown.keyAt(i); |
| 110 | if (test == null || test == lock) continue; |
| 111 | |
| 112 | if (Thread.holdsLock(test)) { |
| 113 | sKnown.valueAt(i).children.add(lock); |
| 114 | } |
| 115 | } |
| 116 | } |
| 117 | |
| 118 | return lock; |
| 119 | } |
| 120 | |
| 121 | /** |
| 122 | * Report the given lock with a well-known label. |
| 123 | */ |
| 124 | public static void installLock(Object lock, String label) { |
| 125 | final LockInfo info = findOrCreateLockInfo(lock); |
| 126 | info.label = label; |
| 127 | } |
| 128 | |
| 129 | private static String lockToString(Object lock) { |
| 130 | final LockInfo info = sKnown.get(lock); |
| 131 | if (info != null) { |
| 132 | return info.label; |
| 133 | } else { |
| 134 | return "0x" + Integer.toHexString(System.identityHashCode(lock)); |
| 135 | } |
| 136 | } |
| 137 | |
| 138 | public static void dump(FileDescriptor fd, PrintWriter pw, String[] args) { |
| 139 | for (int i = 0; i < sKnown.size(); i++) { |
| 140 | final Object lock = sKnown.keyAt(i); |
| 141 | final LockInfo info = sKnown.valueAt(i); |
| 142 | pw.println("Lock " + lockToString(lock) + ":"); |
| 143 | for (int j = 0; j < info.children.size(); j++) { |
| 144 | pw.println(" Child " + lockToString(info.children.valueAt(j))); |
| 145 | } |
| 146 | pw.println(); |
| 147 | } |
| 148 | } |
| 149 | } |