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 <log/log.h>
20
21 #include <errno.h>
22 #include <signal.h>
23 #include <string.h>
24
25 #include <mutex>
26
27 #include "AsynchronousCloseMonitor.h"
28
29 namespace {
30
31 class AsynchronousCloseMonitorImpl {
32 public:
33 explicit AsynchronousCloseMonitorImpl(int fd);
34 ~AsynchronousCloseMonitorImpl();
35 bool wasSignaled() const;
36
37 static void init();
38
39 static void signalBlockedThreads(int fd);
40
41 private:
42 AsynchronousCloseMonitorImpl(const AsynchronousCloseMonitorImpl&) = delete;
43 AsynchronousCloseMonitorImpl& operator=(const AsynchronousCloseMonitorImpl&) = delete;
44
45 AsynchronousCloseMonitorImpl* mPrev;
46 AsynchronousCloseMonitorImpl* mNext;
47 pthread_t mThread;
48 int mFd;
49 bool mSignaled;
50 };
51
52 /**
53 * We use an intrusive doubly-linked list to keep track of blocked threads.
54 * This gives us O(1) insertion and removal, and means we don't need to do any allocation.
55 * (The objects themselves are stack-allocated.)
56 * Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number
57 * of blocked threads (not the number of threads actually blocked on the file descriptor in
58 * question). For now at least, this seems like a good compromise for Android.
59 */
60 static std::mutex blockedThreadListMutex;
61 static AsynchronousCloseMonitorImpl* blockedThreadList = NULL;
62
63 /**
64 * The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN
65 * starts at a higher value.
66 */
67 #if defined(__Fuchsia__)
68 static const int BLOCKED_THREAD_SIGNAL = SIGRTMIN + 2;
69 #elif !defined (__BIONIC__)
70 static const int BLOCKED_THREAD_SIGNAL = SIGRTMAX - 2;
71 #else
72 static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
73 #endif
74
blockedThreadSignalHandler(int)75 static void blockedThreadSignalHandler(int /*signal*/) {
76 // Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
77 }
78
init()79 void AsynchronousCloseMonitorImpl::init() {
80 // Ensure that the signal we send interrupts system calls but doesn't kill threads.
81 // Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
82 // (The whole reason we're sending this signal is to unblock system calls!)
83 struct sigaction sa;
84 memset(&sa, 0, sizeof(sa));
85 sa.sa_handler = blockedThreadSignalHandler;
86 sa.sa_flags = 0;
87 int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
88 if (rc == -1) {
89 ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
90 }
91 }
92
signalBlockedThreads(int fd)93 void AsynchronousCloseMonitorImpl::signalBlockedThreads(int fd) {
94 std::lock_guard<std::mutex> lock(blockedThreadListMutex);
95 for (AsynchronousCloseMonitorImpl* it = blockedThreadList; it != NULL; it = it->mNext) {
96 if (it->mFd == fd) {
97 it->mSignaled = true;
98 pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
99 // Keep going, because there may be more than one thread...
100 }
101 }
102 }
103
wasSignaled() const104 bool AsynchronousCloseMonitorImpl::wasSignaled() const {
105 return mSignaled;
106 }
107
AsynchronousCloseMonitorImpl(int fd)108 AsynchronousCloseMonitorImpl::AsynchronousCloseMonitorImpl(int fd) {
109 std::lock_guard<std::mutex> lock(blockedThreadListMutex);
110 // Who are we, and what are we waiting for?
111 mThread = pthread_self();
112 mFd = fd;
113 mSignaled = false;
114 // Insert ourselves at the head of the intrusive doubly-linked list...
115 mPrev = NULL;
116 mNext = blockedThreadList;
117 if (mNext != NULL) {
118 mNext->mPrev = this;
119 }
120 blockedThreadList = this;
121 }
122
~AsynchronousCloseMonitorImpl()123 AsynchronousCloseMonitorImpl::~AsynchronousCloseMonitorImpl() {
124 std::lock_guard<std::mutex> lock(blockedThreadListMutex);
125 // Unlink ourselves from the intrusive doubly-linked list...
126 if (mNext != NULL) {
127 mNext->mPrev = mPrev;
128 }
129 if (mPrev == NULL) {
130 blockedThreadList = mNext;
131 } else {
132 mPrev->mNext = mNext;
133 }
134 }
135
136 } // namespace
137
138 //
139 // C ABI and API boundary
140 //
141
142 extern "C" {
async_close_monitor_static_init()143 void async_close_monitor_static_init() {
144 AsynchronousCloseMonitorImpl::init();
145 }
146
async_close_monitor_signal_blocked_threads(int fd)147 void async_close_monitor_signal_blocked_threads(int fd) {
148 AsynchronousCloseMonitorImpl::signalBlockedThreads(fd);
149 }
150
async_close_monitor_create(int fd)151 void* async_close_monitor_create(int fd) {
152 return new AsynchronousCloseMonitorImpl(fd);
153 }
154
async_close_monitor_destroy(void * instance)155 void async_close_monitor_destroy(void* instance) {
156 auto monitor = reinterpret_cast<AsynchronousCloseMonitorImpl*>(instance);
157 delete monitor;
158 }
159
async_close_monitor_was_signalled(const void * instance)160 int async_close_monitor_was_signalled(const void* instance) {
161 auto monitor = reinterpret_cast<const AsynchronousCloseMonitorImpl*>(instance);
162 return monitor->wasSignaled() ? 1 : 0;
163 }
164 }
165