In an attempt to rid my GUI of race conditions and deadlocks I have the following function which I call from the c'tor and whenever I need the service which shares my named mutex to provide its input:
void EnvCapt::FireServiceAndOrHold() {
try {
mutTimerSyncEx->ReleaseMutex();
Thread::Sleep(100); //Time enough for the service to complete.
if (!mutTimerSyncEx->WaitOne(3 * int_ms)) {//int_ms = the polling period
//Must've been doubly locked or worse.
mutTimerSyncEx->ReleaseMutex();
FireServiceAndOrHold();
}
} catch (Exception ^ ex) {
//Released unheld mutex. Retake control.
mutTimerSyncEx->WaitOne();
FireServiceAndOrHold();
}
}
This works relatively well but I am calling this before letting the service now I am ready to accept input so it never attempts to wait for me to release the mutex for it. Before I attempt to re-order things I would like to know what is going wrong with the above function. The error I get is:
Object synchronization method was called from an unsynchronized block of code.
Because calling release on a mutex that hasn't been WaitOne'd will throw I catch that, knowing I am free to take ownership of it and continue. But I am wrong. It hangs forever on the WaitOne() statement. I know what the other process is doing all this time because it is trapped in my second debugger window. It is not touching the mutex.
UPDATE
I've attempted the reordering I first suggested, this seemed good but now I find that the mutex is only sort of Global, despite having a Global\name.
It is shared because when my GUI c'tor's it firstInstance is false, hence I attempt to take control of it.
It is not shared because when the GUI calls WaitOne() on it the GUI blocks indefinitely. Whereas the service dances straight through its call to WaitOne() without a care in the world.
I just had an idea what might be going wrong for you there:
Hint: you cannot release a mutex on behalf of the other process! The other process will have to release the mutex if it holds it:
Process 1: Process 2:
============ =============
WaitOne (locks the mutex)
// do work WaitOne (awaits the mutex)
// do more work
// done
ReleaseMutex ------> WaitOne returns from the wait _with
the mutex locked_
Related
Context:
I have a cmd application in java which is written to work in peer-to-peer mode in different servers. Once a server starts, all other instances must stop. So I have written a piece of code that runs in a low priority thread and monitors an AtomicBoolean value autoClose, and whenever autoClose is set to true, thread will close application. (P.S.: I don't want to manually add close because the application has 2 main high priority threads and many temporary normal priority threads).
Here is the code:
/**
* Watches autoClose boolean value and closes the connector once it is true
* <p>
* This is a very low priority thread which continuously monitors autoClose
*/
protected void watchAndClose() {
Thread watchAutoClose = new Thread(() -> {
while (true) {
if (autoClose.get()) {
close();
// wait till closing is successful
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException ignored) {
// I want instance of thread watchAutoClose so I can call this
// watchAutoClose.interrupt();
}
if (!component.getStatus()) setAutoClose(false);
}
}
});
watchAutoClose.setPriority(Thread.MIN_PRIORITY);
watchAutoClose.start();
}
Question:
SonarLint says I can't leave InterruptedException part empty. I have to either throw it again or call thatThread.interrupt().
So how can I do this? I want an instance of thread watchAutoClose inside that thread so I can call watchAutoClose.interrupt(). I tried Thread.currentThread() but I fear with that many threads, the currently executing thread wouldn't be this thread. (i.e, there is a possibility of JVM can choose to switch to another thread by the time it is inside the catch clause and calls Thread.currentThread() so at that time current thread would be the other one and I would interrupt that other thread... correct me if I am too worrying or my concept is totally wrong.)
Or should I ignore the warning altogether and leave catch block?
First of all, it’s not clear why you think that waiting for a second was necessary at all. By the time, the close() method returns, the close() method has been completed. On the other hand, if close() truly triggers some asynchronous action, there is no guaranty that waiting one second will be sufficient for its completion.
Further, addressing your literal question, Thread.currentThread() always return the calling thread’s instance. It’s impossible for a thread to execute that method without being in the running state. When a task switch happens, the thread can’t read the reference at all, until it gets CPU time again. Besides that, since the specification says that this method returns the Thread instance representing the caller, the environment has to ensure this property, regardless of how it implements it. It works even when multiple threads call this method truly at the same time, on different CPU cores.
So, regardless of how questionable the approach of waiting a second is, handling interruption like
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException ignored) {
Thread.currentThread().interrupt();
}
is a valid approach.
But you may also replace this code with
LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(1));
The parkNanos method will return silently on interruption, leaving the calling thread in the interrupted state. So it has the same effect as catching the InterruptedException and restoring the interrupted state, but is simpler and potentially more efficient as no exception needs to be constructed, thrown, and caught.
Another point is that you are creating a polling loop on the atomic variable consuming CPU cycles when the variable is false, which is discouraged, even when you give the thread a low priority.
I am trying to pause my thread waiting for an user action. I know I could use Qt::BlockingQueuedConnection but that is not the point here. I would like to use QWaitCondition but I don't understand in this particular case why I need a QMutex.
Consider this code :
class MyWorker: public QThread
{
private:
QMutex mDummy;
QWaitCondition mStep1;
void doStuff1(){}
void doStuff2(){}
signals:
void step1Finished();
public:
MyWorker(...): {}
protected:
void run()
{
doStuff1();
emit step1Finished();
mDummy.lock();
mStep1.wait(mDummy);
mDummy.unlock();
doStuff2();
}
}
In this case the QMutex mDummy seems useless to me. I use it only because wait() need it as parameter.
I know that wait() unlock the mutex then (re)lock it after waking up, but why there no possibility to use wait() without it?
First of all, wait condition needs a mutex, so you gotta give it one. That's what a wait condition is. It is the most low level signalling mechanism between threads in multi-threading, so it doesn't provide the "convenience" you seem to be looking for.
But you also need the mutex to get things work right. A wait condition might have a spurious wakeup, that is it could be woken up for "no reason" (google "wait condition spurious wakeup" to learn more). So you have to have some condition in there to check, and keep waiting if it's still not time to continue. And to avoid race conditions, that check has to be protected by mutex.
Snippets:
// wait
mDummy.lock();
mStopWaiting = false; // maybe here, if you want to make sure this waits in all cases
while (!mStopWaiting)
{
// note that wait releases the mutex while waiting
mStep1.wait(&mDummy);
}
mDummy.unlock();
// signal end of wait
mDummy.lock();
mStopWaiting = true;
mStep1.wakeOne(); // or wakeAll() maybe depending on other code
mDummy.unlock();
As you can see, that mutex isn't so dummy after all. Note that all access to mStopWaiting has to be protected by this mutex, not just here.
Imagine you want to wait for something to happen. Since that something has to happen in another thread (since this thread is waiting) it has to be protected in some way to avoid race conditions.
Imagine you use the following code:
Acquire a lock.
Check if the thing you want to wait for has happened.
If it has, stop, you're done.
If it hasn't, wait.
Oops. We're still holding the lock. There's no way the thing we're waiting for can happen because no other thread can access it.
Let's try again.
Acquire a lock.
Check if the thing you want to wait for has happened.
If it has, stop, you're done.
If it hasn't, release the lock and wait.
Oops. What if after we release the lock but before we wait, it happens. Then we'll be waiting for something that already happened.
So what we need for step 4 is an atomic "unlock and wait" operation. This releases the lock and waits without giving another thread a chance to sneak in and change things before we can start waiting.
If you don't need an atomic "unlock and wait" operation, don't use QWaitCondition. This is its sole purpose. It takes a QMutex so it knows what to unlock. That QMutex must protect whatever it is the thread is waiting for or your code will be vulnerable to the very race condition QWaitCondition exists to solve for you.
TL;DR
Why does std::condition_variable::wait needs a mutex as one of its variables?
Answer 1
You may look a the documentation and quote that:
wait... Atomically releases lock
But that's not a real reason. That's just validate my question even more: why does it need it in the first place?
Answer 2
predicate is most likely query the state of a shared resource and it must be lock guarded.
OK. fair.
Two questions here
Is it always true that predicate query the state of a shared resource? I assume yes. I t doesn't make sense to me to implement it otherwise
What if I do not pass any predicate (it is optional)?
Using predicate - lock makes sense
int i = 0;
void waits()
{
std::unique_lock<std::mutex> lk(cv_m);
cv.wait(lk, []{return i == 1;});
std::cout << i;
}
Not Using predicate - why can't we lock after the wait?
int i = 0;
void waits()
{
cv.wait(lk);
std::unique_lock<std::mutex> lk(cv_m);
std::cout << i;
}
Notes
I know that there are no harmful implications to this practice. I just don't know how to explain to my self why it was design this way?
Question
If predicate is optional and is not passed to wait, why do we need the lock?
When using a condition variable to wait for a condition, a thread performs the following sequence of steps:
It determines that the condition is not currently true.
It starts waiting for some other thread to make the condition true. This is the wait call.
For example, the condition might be that a queue has elements in it, and a thread might see that the queue is empty and wait for another thread to put things in the queue.
If another thread were to intercede between these two steps, it could make the condition true and notify on the condition variable before the first thread actually starts waiting. In this case, the waiting thread would not receive the notification, and it might never stop waiting.
The purpose of requiring the lock to be held is to prevent other threads from interceding like this. Additionally, the lock must be unlocked to allow other threads to do whatever we're waiting for, but it can't happen before the wait call because of the notify-before-wait problem, and it can't happen after the wait call because we can't do anything while we're waiting. It has to be part of the wait call, so wait has to know about the lock.
Now, you might look at the notify_* methods and notice that those methods don't require the lock to be held, so there's nothing actually stopping another thread from notifying between steps 1 and 2. However, a thread calling notify_* is supposed to hold the lock while performing whatever action it does to make the condition true, which is usually enough protection.
TL;DR
If predicate is optional and is not passed to wait, why do we need the lock?
condition_variable is designed to wait for a certain condition to come true, not to wait just for a notification. So to "catch" the "moment" when the condition becomes true you need to check the condition and wait for the notification. And to avoid a race condition you need those two to be a single atomic operation.
Purpose Of condition_variable:
Enable a program to implement this: do some action when a condition C holds.
Intended Protocol:
Condition producer changes state of the world from !C to C.
Condition consumer waits for C to happen and takes the action while/after condition C holds.
Simplification:
For simplicity (to limit number of cases to think of) let's assume that C never switches back to !C. Let's also forget about spurious wakeups. Even with this assumptions we'll see that the lock is necessary.
Naive Approach:
Let's have two threads with an essential code summarized like this:
void producer() {
_condition = true;
_condition_variable.notify_all();
}
void consumer() {
if (!_condition) {
_condition_variable.wait();
}
action();
}
The Problem:
The problem here is a race condition. A problematic interleaving of the threads is following:
The consumer reads condition, checks it to be false and decides to wait.
A thread scheduler interrupts consumer and resumes producer.
The producer updates condition to become true and invokes notify_all().
The consumer is resumed.
The consumer actually does wait(), but is never notified and waken up (a liveness hazard).
So without locking the consumer may miss the event of the condition becoming true.
Solution:
Disclaimer: this code still does not handle spurious wakeups and possibility of condition becoming false again.
void producer() {
{ std::unique_lock<std::mutex> l(_mutex);
_condition = true;
}
_condition_variable.notify_all();
}
void consumer() {
{ std::unique_lock<std::mutex> l(_mutex);
if (!_condition) {
_condition_variable.wait(l);
}
}
action();
}
Here we check condition, release lock and start waiting as a single atomic operation, preventing the race condition mentioned before.
See Also
Why Lock condition await must hold the lock
You need a std::unique_lock when using std::condition_variable for the same reason you need a std::FILE* when using std::fwrite and for the same reason a BasicLockable is necessary when using std::unique_lock itself.
The feature std::fwrite gives you, entire the reason it exists, is to write to files. So you have to give it a file. The feature std::unique_lock provides you is RAII locking and unlocking of a mutex (or another BasicLockable, like std::shared_mutex, etc.) so you have to give it something to lock and unlock.
The feature std::condition_variable provides, the entire reason it exists, is the atomically waiting and unlocking a lock (and completing a wait and locking). So you have to give it something to lock.
Why would someone want that is a separate question that has been discussed already. For example:
When is a condition variable needed, isn't a mutex enough?
Conditional Variable vs Semaphore
Advantages of using condition variables over mutex
And so on.
As has been explained, the pred parameter is optional, but having some sort of a predicate and testing it isn't. Or, in other words, not having a predicate doesn't make any sense inn a manner similar to how having a condition variable without a lock doesn't making any sense.
The reason you have a lock is because you have shared state you need to protect from simultaneous access. Some function of that shared state is the predicate.
If you don't have a predicate and you don't have a lock you really don't need a condition variable just like if you don't have a file you really don't need fwrite.
A final point is that the second code snippet you wrote is very broken. Obviously it won't compile as you define the lock after you try to pass it as an argument to condition_variable::wait(). You probably meant something like:
std::mutex mtx_cv;
std::condition_variable cv;
...
{
std::unique_lock<std::mutex> lk(mtx_cv);
cv.wait(lk);
lk.lock(); // throws std::system_error with an error code of std::errc::resource_deadlock_would_occur
}
The reason this is wrong is very simple. condition_variable::wait's effects are (from [thread.condition.condvar]):
Effects:
— Atomically calls lock.unlock() and blocks on *this.
— When unblocked, calls lock.lock() (possibly blocking on the lock), then returns.
— The function will unblock when signaled by a call to notify_one() or a call to notify_all(), or spuriously
After the return from wait() the lock is locked, and unique_lock::lock() throws an exception if it has already locked the mutex it wraps ([thread.lock.unique.locking]).
Again, why would someone want coupling waiting and locking the way std::condition_variable does is a separate question, but given that it does - you cannot, by definition, lock a std::condition_variable's std::unique_lock after std::condition_variable::wait has returned.
It's not stated in the documentation (and could be implemented differently) but conceptually you can imagine the condition variable has another mutex to both protect its own data but also coordinate the condition, waiting and notification with modification of the consumer code data (e.g. queue.size()) affecting the test.
So when you call wait(...) the following (logically) happens.
Precondition: The consumer code holds the lock (CCL) controlling the consumer condition data (CCD).
The condition is checked, if true, execution in the consumer code continues still holding the lock.
If false, it first acquires its own lock (CVL), adds the current thread to the waiting thread collection releases the consumer lock and puts itself to waiting and releases its own lock (CVL).
That final step is tricky because it needs to sleep the thread and release the CVL at the same time or in that order or in a way that threads notified just before going to wait are able to (somehow) not go to wait.
The step of acquiring the CVL before releasing the CCD is key. Any parallel thread trying to update the CCD and notify will be blocked either by the CCL or CVL. If the CCL was released before acquiring the CVL a parallel thread could acquire the CCL, change the data and then notify before the the to-be-waiting thread is added to the waiters.
A parallel thread acquires the CCL, modifies the data to make the condition true (or at least worth testing) and then notifies. Notification acquires the the CVL and identifies a blocked thread (or threads) if any to unwait. The unwaited threads then seek to acquire the CCL and may block there but won't leave wait and re-perform the test until they've acquired it.
Notification must acquire the CVL to make sure threads that have found the test false have been added to the waiters.
It's OK (possibly preferable for performance) to notify without holding the CCL because the hand-off between the CCL and CVL in the wait code is ensuring the ordering.
It may be preferrable because notifying when holding the CCL may mean all the unwaited threads just unwait to block (on the CCL) while the thread modifying the data is still holding the lock.
Notice that even if the CCD is atomic you must modify it holding the CCL or that Lock CVL, unlock CCL step won't ensure the total ordering required to make sure notifications aren't sent when threads are in the process of going to wait.
The standard only talks about atomicity of operations and another implementation may have a way of blocking notification before completing the 'add to waiters' step has completed following a failed test. The C++ Standard is careful to not dictate an implementation.
In all that, to answer some of the specific questions.
Must the state be shared? Sort of. There could be an external condition like a file being in a directory and the wait is timed to re-try after a time-period. You can decide for yourself whether you consider the file system or even just the wall-clock to be shared state.
Must there be any state? Not necessarily. A thread can wait on notification.
That could be tricky to coordinate because there has to be enough sequencing to stop the other thread notifying out of turn. The commonest solution is to have some boolean flag set by the notifying thread so the notified thread knows if it missed it. The normal use of void wait(std::unique_lock<std::mutex>& lk) is when the predicate is checked outside:
std::unique_lock<std::mutex> ulk(ccd_mutex)
while(!condition){
cv.wait(ulk);
}
Where the notifying thread uses:
{
std::lock_guard<std::mutex> guard(ccd_mutex);
condition=true;
}
cv.notify();
The reason is that in some times the waiting-thread holds the m_mutex:
#include <mutex>
#include <condition_variable>
void CMyClass::MyFunc()
{
std::unique_lock<std::mutex> guard(m_mutex);
// do something (on the protected resource)
m_condiotion.wait(guard, [this]() {return !m_bSpuriousWake; });
// do something else (on the protected resource)
guard.unluck();
// do something else than else
}
and a thread should never go to sleep while holding a m_mutex. One doesn't want to lock everybody out, while sleeping. So, atomically: {guard is unlocked and the thread go to sleep}. Once it waked up by the other-thread (m_condiotion.notify_one(), let's say) guard is locked again, and then the thread continue.
Reference (video)
Because if not so, there's a race condition before the waiting thread noticing the change of the shared state and the wait() call.
Assume we got a shared state of type std::atomic state_, there's still a fair chance for the waiting thread to miss a notification:
T1(waiting) | T2(notification)
---------------------------------------------- * ---------------------------
1) for (int i = state_; i != 0; i = state_) { |
2) | state_ = 0;
3) | cv.notify();
4) cv.wait(); |
5) }
6) // go on with the satisfied condition... |
Note that the wait() call failed to notice the latest value of state_ and may keep waiting forever.
I have this pretty straightforward piece of code that very rarely throws "System.ApplicationException : Object synchronization method was called from an unsynchronized block of code." when ReleaseMutex() is called.
I logically analyzed the flow of the method and just cannot understand how/why this could happen.
To my understanding, the ownership of mutex is guaranteed in this case:
readonly string mutexKey;
public Logger(string dbServer, string dbName)
{
this.mutexKey = ServiceManagerHelper.GetServiceName(dbServer, dbName);
}
private void Log(LogType type, string message, Exception ex)
{
using (var mutex = new Mutex(false, mutexKey))
{
bool acquiredMutex;
try
{
acquiredMutex = mutex.WaitOne(TimeSpan.FromSeconds(5));
}
catch (AbandonedMutexException)
{
acquiredMutex = true;
}
if (acquiredMutex)
{
try
{
// some application code here
}
finally
{
mutex.ReleaseMutex();
}
}
}
}
catch (AbandonedMutexException)
{
acquiredMutex = true;
}
This is a very serious bug in your code. Catching an AbandonedMutexException is never correct, it is a very serious mishap. Another thread acquired the mutex but terminated without calling ReleaseMutex(). You've irrecoverably lost synchronization and the mutex is no longer usable.
You were sort of lucky by making a mistake and assuming that you acquired the mutex anyway. You didn't. The ReleaseMutex() call will now bomb with the exception you quoted.
You cannot recover from this mishap, other than by terminating the program (the wise choice) or by disabling logging completely so the mutex will never be used again. Make the wise choice by removing the catch clause. Discovering the true source of the problem, that thread that crashed and didn't call ReleaseMutex(), is out of context for this question, there are no hints. You've been ignoring this problem, papered it over by catching AME, you can't ignore it.
In my case, i see the same behavior like Nathan Schubkegel. I use await's, and Thread.CurrentThread.ManagedThreadId gives another value for the "same" thread. I mean, thread was started with ManagedThreadId == 10, and Mutex was owned with this thread id, but later ReleaseMutex() causes ApplicationException with message: "Object synchronization method was called from an unsynchronized block of code", and i see that ManagedThreadId == 11 at this time :) . It seems, await sometimes changes thread id when returns. It seems, that is the reason. Mutex thinks that another thread wants to release it. It's sad, that Mutex documentation does not make ATTENTION on this moment.
So, you CAN NOT use asynchronous operator await between Mutex acquire and release. It's because C# compiler replaces plain operator await by asynchronous callback, and this callback can be made by ANOTHER thread. Usually, it's the same thread, but sometimes it's another thread (from thread pool).
Mutex checks thread. Only thread that acquired Mutex may release it. If you need synchronization without this checking, use Semaphore. SemaphoreSlim has asynchronous method WaitAsync() - it's cool.
This exception is raised when you call ReleaseMutex() from a thread that does not own the mutex. Search // some application code here for code that releases the mutex.
Also reconsider whether you're actually calling ReleaseMutex() from the same thread where you called WaitOne(). Example: I arrived at this post because I was using async/await and my code resumed on a different thread and tried to release a mutex the thread didn't own.
what is the rigth way to close Thread in Winapi, threads don't use common resources.
I am creating threads with CreateThread , but I don't know how to close it correctly in ,because someone suggest to use TerminateThread , others ExitThread , but what is the correct way to close it .
Also where should I call closing function in WM_CLOSE or WM_DESTROY ?
Thx in advance .
The "nicest" way to close a thread in Windows is by "telling" the thread to shutdown via some thread-safe signaling mechanism, then simply letting it reach its demise its own, potentially waiting for it to do so via one of the WaitForXXXX functions if completion detection is needed (which is frequently the case). Something like:
Main thread:
// some global event all threads can reach
ghStopEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
// create the child thread
hThread = CreateThread(NULL, 0, ThreadProc, NULL, 0, NULL);
//
// ... continue other work.
//
// tell thread to stop
SetEvent(ghStopEvent);
// now wait for thread to signal termination
WaitForSingleObject(hThread, INFINITE);
// important. close handles when no longer needed
CloseHandle(hThread);
CloseHandle(ghStopEvent);
Child thread:
DWORD WINAPI ThreadProc(LPVOID pv)
{
// do threaded work
while (WaitForSingleObject(ghStopEvent, 1) == WAIT_TIMEOUT)
{
// do thread busy work
}
return 0;
}
Obviously things can get a lot more complicated once you start putting it in practice. If by "common" resources you mean something like the ghStopEvent in the prior example, it becomes considerably more difficult. Terminating a child thread via TerminateThread is strongly discouraged because there is no logical cleanup performed at all. The warnings specified in the `TerminateThread documentation are self-explanatory, and should be heeded. With great power comes....
Finally, even the called thread invoking ExitThread is not required explicitly by you, and though you can do so, I strongly advise against it in C++ programs. It is called for you once the thread procedure logically returns from the ThreadProc. I prefer the model above simply because it is dead-easy to implement and supports full RAII of C++ object cleanup, which neither ExitThread nor TerminateThread provide. For example, the ExitThread documentation :
...in C++ code, the thread is exited before any destructors can be called
or any other automatic cleanup can be performed. Therefore, in C++
code, you should return from your thread function.
Anyway, start simple. Get a handle on things with super-simple examples, then work your way up from there. There are a ton of multi-threaded examples on the web, Learn from the good ones and challenge yourself to identify the bad ones.
Best of luck.
So you need to figure out what sort of behaviour you need to have.
Following is a simple description of the methods taken from documentation:
"TerminateThread is a dangerous function that should only be used in the most extreme cases. You should call TerminateThread only if you know exactly what the target thread is doing, and you control all of the code that the target thread could possibly be running at the time of the termination. For example, TerminateThread can result in the following problems:
If the target thread owns a critical section, the critical section will not be released.
If the target thread is allocating memory from the heap, the heap lock will not be released.
If the target thread is executing certain kernel32 calls when it is terminated, the kernel32 state for the thread's process could be inconsistent.
If the target thread is manipulating the global state of a shared DLL, the state of the DLL could be destroyed, affecting other users of the DLL."
So if you need your thread to terminate at any cost, call this method.
About ExitThread, this is more graceful. By calling ExitThread, you're telling to windows you're done with that calling thread, so the rest of the code isn't going to get called. It's a bit like calling exit(0).
"ExitThread is the preferred method of exiting a thread. When this function is called (either explicitly or by returning from a thread procedure), the current thread's stack is deallocated, all pending I/O initiated by the thread is canceled, and the thread terminates. If the thread is the last thread in the process when this function is called, the thread's process is also terminated."