blob: 6c9b2e58999ac1fbaf3fe6dc06360dae01bb0214 [file] [log] [blame]
* Copyright (C) 2010 The Android Open Source Project
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#define LOG_TAG "AsynchronousCloseMonitor"
#include "AsynchronousCloseMonitor.h"
#include "cutils/log.h"
#include <errno.h>
#include <signal.h>
#include <string.h>
* We use an intrusive doubly-linked list to keep track of blocked threads.
* This gives us O(1) insertion and removal, and means we don't need to do any allocation.
* (The objects themselves are stack-allocated.)
* Waking potentially-blocked threads when a file descriptor is closed is O(n) in the total number
* of blocked threads (not the number of threads actually blocked on the file descriptor in
* question). For now at least, this seems like a good compromise for Android.
static pthread_mutex_t blockedThreadListMutex = PTHREAD_MUTEX_INITIALIZER;
static AsynchronousCloseMonitor* blockedThreadList = NULL;
* The specific signal chosen here is arbitrary, but bionic needs to know so that SIGRTMIN
* starts at a higher value.
static const int BLOCKED_THREAD_SIGNAL = __SIGRTMIN + 2;
static void blockedThreadSignalHandler(int /*signal*/) {
// Do nothing. We only sent this signal for its side-effect of interrupting syscalls.
void AsynchronousCloseMonitor::init() {
// Ensure that the signal we send interrupts system calls but doesn't kill threads.
// Using sigaction(2) lets us ensure that the SA_RESTART flag is not set.
// (The whole reason we're sending this signal is to unblock system calls!)
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = blockedThreadSignalHandler;
sa.sa_flags = 0;
int rc = sigaction(BLOCKED_THREAD_SIGNAL, &sa, NULL);
if (rc == -1) {
ALOGE("setting blocked thread signal handler failed: %s", strerror(errno));
void AsynchronousCloseMonitor::signalBlockedThreads(int fd) {
ScopedPthreadMutexLock lock(&blockedThreadListMutex);
for (AsynchronousCloseMonitor* it = blockedThreadList; it != NULL; it = it->mNext) {
if (it->mFd == fd) {
it->mSignaled = true;
pthread_kill(it->mThread, BLOCKED_THREAD_SIGNAL);
// Keep going, because there may be more than one thread...
bool AsynchronousCloseMonitor::wasSignaled() const {
return mSignaled;
AsynchronousCloseMonitor::AsynchronousCloseMonitor(int fd) {
ScopedPthreadMutexLock lock(&blockedThreadListMutex);
// Who are we, and what are we waiting for?
mThread = pthread_self();
mFd = fd;
mSignaled = false;
// Insert ourselves at the head of the intrusive doubly-linked list...
mPrev = NULL;
mNext = blockedThreadList;
if (mNext != NULL) {
mNext->mPrev = this;
blockedThreadList = this;
AsynchronousCloseMonitor::~AsynchronousCloseMonitor() {
ScopedPthreadMutexLock lock(&blockedThreadListMutex);
// Unlink ourselves from the intrusive doubly-linked list...
if (mNext != NULL) {
mNext->mPrev = mPrev;
if (mPrev == NULL) {
blockedThreadList = mNext;
} else {
mPrev->mNext = mNext;