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 /**
27 * We use an intrusive doubly-linked list to keep track of blocked threads.
28 * This gives us O(1) insertion and removal, and means we don't need to do any allocation.
29 * (The objects themselves are stack-allocated.)
30 * Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number
31 * of blocked threads (not the number of threads actually blocked on the file descriptor in
32 * question). For now at least, this seems like a good compromise for Android.
33 */
34 static pthread_mutex_t blockedThreadListMutex = PTHREAD_MUTEX_INITIALIZER;
35 static AsynchronousCloseMonitor* blockedThreadList = NULL;
36
37 /**
38 * The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN
39 * starts at a higher value.
40 */
41 #if defined(__APPLE__)
42 static const int BLOCKED_THREAD_SIGNAL = SIGUSR2;
43 #else
44 static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
45 #endif
46
blockedThreadSignalHandler(int)47 static void blockedThreadSignalHandler(int /*signal*/) {
48 // Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
49 }
50
init()51 void AsynchronousCloseMonitor::init() {
52 // Ensure that the signal we send interrupts system calls but doesn't kill threads.
53 // Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
54 // (The whole reason we're sending this signal is to unblock system calls!)
55 struct sigaction sa;
56 memset(&sa, 0, sizeof(sa));
57 sa.sa_handler = blockedThreadSignalHandler;
58 sa.sa_flags = 0;
59 int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
60 if (rc == -1) {
61 ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
62 }
63 }
64
signalBlockedThreads(int fd)65 void AsynchronousCloseMonitor::signalBlockedThreads(int fd) {
66 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
67 for (AsynchronousCloseMonitor* it = blockedThreadList; it != NULL; it = it->mNext) {
68 if (it->mFd == fd) {
69 it->mSignaled = true;
70 pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
71 // Keep going, because there may be more than one thread...
72 }
73 }
74 }
75
wasSignaled() const76 bool AsynchronousCloseMonitor::wasSignaled() const {
77 return mSignaled;
78 }
79
AsynchronousCloseMonitor(int fd)80 AsynchronousCloseMonitor::AsynchronousCloseMonitor(int fd) {
81 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
82 // Who are we, and what are we waiting for?
83 mThread = pthread_self();
84 mFd = fd;
85 mSignaled = false;
86 // Insert ourselves at the head of the intrusive doubly-linked list...
87 mPrev = NULL;
88 mNext = blockedThreadList;
89 if (mNext != NULL) {
90 mNext->mPrev = this;
91 }
92 blockedThreadList = this;
93 }
94
~AsynchronousCloseMonitor()95 AsynchronousCloseMonitor::~AsynchronousCloseMonitor() {
96 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
97 // Unlink ourselves from the intrusive doubly-linked list...
98 if (mNext != NULL) {
99 mNext->mPrev = mPrev;
100 }
101 if (mPrev == NULL) {
102 blockedThreadList = mNext;
103 } else {
104 mPrev->mNext = mNext;
105 }
106 }
107