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 <nativehelper/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
blockedThreadSignalHandler(int)45 static void blockedThreadSignalHandler(int /*signal*/) {
46 // Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
47 }
48
init()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
signalBlockedThreads(int fd)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
wasSignaled() const74 bool AsynchronousCloseMonitor::wasSignaled() const {
75 return mSignaled;
76 }
77
AsynchronousCloseMonitor(int fd)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
~AsynchronousCloseMonitor()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