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 static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
42
blockedThreadSignalHandler(int)43 static void blockedThreadSignalHandler(int /*signal*/) {
44 // Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
45 }
46
init()47 void AsynchronousCloseMonitor::init() {
48 // Ensure that the signal we send interrupts system calls but doesn't kill threads.
49 // Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
50 // (The whole reason we're sending this signal is to unblock system calls!)
51 struct sigaction sa;
52 memset(&sa, 0, sizeof(sa));
53 sa.sa_handler = blockedThreadSignalHandler;
54 sa.sa_flags = 0;
55 int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
56 if (rc == -1) {
57 ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
58 }
59 }
60
signalBlockedThreads(int fd)61 void AsynchronousCloseMonitor::signalBlockedThreads(int fd) {
62 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
63 for (AsynchronousCloseMonitor* it = blockedThreadList; it != NULL; it = it->mNext) {
64 if (it->mFd == fd) {
65 it->mSignaled = true;
66 pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
67 // Keep going, because there may be more than one thread...
68 }
69 }
70 }
71
wasSignaled() const72 bool AsynchronousCloseMonitor::wasSignaled() const {
73 return mSignaled;
74 }
75
AsynchronousCloseMonitor(int fd)76 AsynchronousCloseMonitor::AsynchronousCloseMonitor(int fd) {
77 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
78 // Who are we, and what are we waiting for?
79 mThread = pthread_self();
80 mFd = fd;
81 mSignaled = false;
82 // Insert ourselves at the head of the intrusive doubly-linked list...
83 mPrev = NULL;
84 mNext = blockedThreadList;
85 if (mNext != NULL) {
86 mNext->mPrev = this;
87 }
88 blockedThreadList = this;
89 }
90
~AsynchronousCloseMonitor()91 AsynchronousCloseMonitor::~AsynchronousCloseMonitor() {
92 ScopedPthreadMutexLock lock(&blockedThreadListMutex);
93 // Unlink ourselves from the intrusive doubly-linked list...
94 if (mNext != NULL) {
95 mNext->mPrev = mPrev;
96 }
97 if (mPrev == NULL) {
98 blockedThreadList = mNext;
99 } else {
100 mPrev->mNext = mNext;
101 }
102 }
103