1 /* 2 * Copyright (C) 2010 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 #define LOG_TAG "AsynchronousCloseMonitor" 18 19 #include "AsynchronousCloseMonitor.h" 20 #include "cutils/log.h" 21 22 #include <errno.h> 23 #include <signal.h> 24 #include <string.h> 25 26 #include <mutex> 27 28 /** 29 * We use an intrusive doubly-linked list to keep track of blocked threads. 30 * This gives us O(1) insertion and removal, and means we don't need to do any allocation. 31 * (The objects themselves are stack-allocated.) 32 * Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number 33 * of blocked threads (not the number of threads actually blocked on the file descriptor in 34 * question). For now at least, this seems like a good compromise for Android. 35 */ 36 static std::mutex blockedThreadListMutex; 37 static AsynchronousCloseMonitor* blockedThreadList = NULL; 38 39 /** 40 * The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN 41 * starts at a higher value. 42 */ 43 static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2; 44 45 static void blockedThreadSignalHandler(int /*signal*/) { 46 // Do nothing. We only sent this signal for its side-effect of interrupting syscalls. 47 } 48 49 void AsynchronousCloseMonitor::init() { 50 // Ensure that the signal we send interrupts system calls but doesn't kill threads. 51 // Using sigaction(2) lets us ensure that the SA_RESTART flag is not set. 52 // (The whole reason we're sending this signal is to unblock system calls!) 53 struct sigaction sa; 54 memset(&sa, 0, sizeof(sa)); 55 sa.sa_handler = blockedThreadSignalHandler; 56 sa.sa_flags = 0; 57 int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL); 58 if (rc == -1) { 59 ALOGE("setting blocked thread signal handler failed: %s", strerror(errno)); 60 } 61 } 62 63 void AsynchronousCloseMonitor::signalBlockedThreads(int fd) { 64 std::lock_guard<std::mutex> lock(blockedThreadListMutex); 65 for (AsynchronousCloseMonitor* it = blockedThreadList; it != NULL; it = it->mNext) { 66 if (it->mFd == fd) { 67 it->mSignaled = true; 68 pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL); 69 // Keep going, because there may be more than one thread... 70 } 71 } 72 } 73 74 bool AsynchronousCloseMonitor::wasSignaled() const { 75 return mSignaled; 76 } 77 78 AsynchronousCloseMonitor::AsynchronousCloseMonitor(int fd) { 79 std::lock_guard<std::mutex> lock(blockedThreadListMutex); 80 // Who are we, and what are we waiting for? 81 mThread = pthread_self(); 82 mFd = fd; 83 mSignaled = false; 84 // Insert ourselves at the head of the intrusive doubly-linked list... 85 mPrev = NULL; 86 mNext = blockedThreadList; 87 if (mNext != NULL) { 88 mNext->mPrev = this; 89 } 90 blockedThreadList = this; 91 } 92 93 AsynchronousCloseMonitor::~AsynchronousCloseMonitor() { 94 std::lock_guard<std::mutex> lock(blockedThreadListMutex); 95 // Unlink ourselves from the intrusive doubly-linked list... 96 if (mNext != NULL) { 97 mNext->mPrev = mPrev; 98 } 99 if (mPrev == NULL) { 100 blockedThreadList = mNext; 101 } else { 102 mPrev->mNext = mNext; 103 } 104 } 105
