While implementing a thread pool pattern in C++ based on this, I came across a few questions.
Let's assume minimal code sample:
std::mutex thread_mutex;
std::condition_variable thread_condition;
void thread_func() {
std::unique_lock<std::mutex> lock(thread_mutex);
thread_condition.wait(lock);
lock.unlock();
}
std::thread t1 = std::thread(thread_func);
Regarding cppreference.com about conditon_variable::wait(), wait() causes the current thread to block. What is locking the mutex then for when I only need one thread at all using wait() to get notified when something is to do?
unique_lock will block the thread when the mutex already has been locked by another thread. But this wouldn't be neccesary as long as wait() blocks anyway or what do I miss here?
Adding a few lines at the bottom...
std::thread t2 = std::thread(thread_func);
thread_condition.notify_all()
When unique_lock is blocking the thread, how will notify_all() reach both threads when one of them is locked by unique_lock and the other is blocked by wait()? I understand that blocking wait() will be freed by notify_all() which afterwards leads to unlocking the mutex and that this gives chance to the other thread for locking first the mutex and blocking thread by wait() afterwards. But how is this thread notified than?
Expanding this question by adding a loop in thread_func()...
std::mutex thread_mutex;
std::condition_variable thread_condition;
void thread_func() {
while(true) {
std::unique_lock<std::mutex> lock(thread_mutex);
thread_condition.wait(lock);
lock.unlock();
}
}
std::thread t1 = std::thread(thread_func);
std::thread t2 = std::thread(thread_func);
thread_condition.notify_all()
While reading documentation, I would now expect both threads running endlessly. But they do not return from wait() lock. Why do I have to use a predicate for expected behaviour like this:
bool wakeup = false;
//[...]
thread_condition.wait(lock, [] { return wakeup; });
//[...]
wakeup = !wakeup;
thread_condition.notify_all();
Thanks in advance.
This is really close to being a duplicate, but it's actually that question that answers this one; we also have an answer that more or less answers this question, but the question is distinct. I think that an independent answer is needed, even though it's little more than a (long) definition.
What is a condition variable?
The operational definition is that it's a means for a thread to block until a message arrives from another thread. A mutex alone can't possibly do this: if all other threads are busy with unrelated work, a mutex can't block a thread at all. A semaphore can block a lone thread, but it's tightly bound to the notion of a count, which isn't always appropriate to the nature of the message to receive.
This "channel" can be implemented in several ways. Very low-tech is to use a pipe, but that involves expensive system calls. Windows provides the Event object which is fundamentally a boolean on whose truth a thread may wait. (C++20 provides a similar feature with atomic_flag::wait.)
Condition variables take a different approach: their structural definition is that they are stateless, but have a special connection to a corresponding mutex type. The latter is necessitated by the former: without state, it is impossible to store a message, so arrangements must be made to prevent sending a message during some interval between a thread recognizing the need to wait (by examining some other state: perhaps that the queue from which it wants to pop is empty) and it actually being blocked. Of course, after the thread is blocked it cannot take any action to allow the message to be sent, so the condition variable must do so.
This is implemented by having the thread take a mutex before checking the condition and having wait release that mutex only after the thread can receive the message. (In some implementations, the mutex is also used to protect the workings of the condition variable, but C++ does not do so.) When the message is received, the mutex is re-acquired (which may block the thread again for a time), as is necessary to consult the external state again. wait thus acts like an everted std::unique_lock: the mutex is unlocked during wait and locked again afterwards, with possibly arbitary changes having been made by other threads in the meantime.
Answers
Given this understanding, the individual answers here are trivial:
Locking the mutex allows the waiting thread to safely decide to wait, given that there must be some other thread affecting the state in question.
If the std::unique_lock blocks, some other thread is currently updating the state, which might actually obviate the need for wait.
Any number of threads can be in wait, since each unlocks the mutex when it calls it.
Waiting on a condition variable, er, unconditionally is always wrong: the state you're after might already apply, with no further messages coming.
Related
I am in process of implementing messages passing from one thread to another
Thread 1: Callback functions are registered with libraries, on callback, functions are invoked and needs to be send to another thread for processing as it takes time.
Thread 2: Thread to check if any messages are available(preferrednas in queue) and process the same.
Is condition_variable usage with mutex a correct approach to start considering thread 2 processing takes time in which multiple other messages can be added by thread 1?
Is condition_variable usage with mutex a correct approach to start considering thread 2 processing takes time in which multiple other messages can be added by thread 1?
The question is a bit vague about how a condition variable and mutex would be used, but yes, there would definitely be a role for such objects. The high-level view would be something like this:
The mutex would protect access to the message queue. Any read or modification of the queue, by any thread, would be done only while holding the mutex locked.
The message-processing thread would block on the CV in the event that it became ready to process a new message but the queue was empty.
The message-generating thread would signal the CV each time it enqueued a new message.
This is exactly a producer / consumer problem, and you can find a lot of information about such problems using that terminology.
But note also that there are multiple message queue implementations already available to serve exactly your purpose ("message queue" is in fact a standard term for these), so you should consider whether you really want to reinvent this wheel.
In general, mutexes are intended to control access between threads; but not great for notifying between threads.
If you design Thread2 to wait on the condition; you can simply process messages as they are received from Thread1.
Here would be a rough implementation
void pushFunction
{
// Obtain the mutex (preferrably scoped lock in boost or c++17)
std::lock_guard lock(myMutex);
const bool empty = myQueue.empty();
myQueue.push(data);
lock.unlock();
if(empty)
{
conditionVar.notify_one();
}
}
In Thread 2
void waitForMessage()
{
std::lock_guard lock(myMutex);
while (myQueue.empty())
{
conditionVar.wait(lock);
}
rxMessage = myQueue.front();
myQueue.pop();
}
It's important to note that the condition can spuriously wake up so it's important to keep it in the 'while empty' loop.
See https://en.cppreference.com/w/cpp/thread/condition_variable
I’m reading up on pthread.h; the condition variable related functions (like pthread_cond_wait(3)) require a mutex as an argument. Why? As far as I can tell, I’m going to be creating a mutex just to use as that argument? What is that mutex supposed to do?
It's just the way that condition variables are (or were originally) implemented.
The mutex is used to protect the condition variable itself. That's why you need it locked before you do a wait.
The wait will "atomically" unlock the mutex, allowing others access to the condition variable (for signalling). Then when the condition variable is signalled or broadcast to, one or more of the threads on the waiting list will be woken up and the mutex will be magically locked again for that thread.
You typically see the following operation with condition variables, illustrating how they work. The following example is a worker thread which is given work via a signal to a condition variable.
thread:
initialise.
lock mutex.
while thread not told to stop working:
wait on condvar using mutex.
if work is available to be done:
do the work.
unlock mutex.
clean up.
exit thread.
The work is done within this loop provided that there is some available when the wait returns. When the thread has been flagged to stop doing work (usually by another thread setting the exit condition then kicking the condition variable to wake this thread up), the loop will exit, the mutex will be unlocked and this thread will exit.
The code above is a single-consumer model as the mutex remains locked while the work is being done. For a multi-consumer variation, you can use, as an example:
thread:
initialise.
lock mutex.
while thread not told to stop working:
wait on condvar using mutex.
if work is available to be done:
copy work to thread local storage.
unlock mutex.
do the work.
lock mutex.
unlock mutex.
clean up.
exit thread.
which allows other consumers to receive work while this one is doing work.
The condition variable relieves you of the burden of polling some condition instead allowing another thread to notify you when something needs to happen. Another thread can tell that thread that work is available as follows:
lock mutex.
flag work as available.
signal condition variable.
unlock mutex.
The vast majority of what are often erroneously called spurious wakeups was generally always because multiple threads had been signalled within their pthread_cond_wait call (broadcast), one would return with the mutex, do the work, then re-wait.
Then the second signalled thread could come out when there was no work to be done. So you had to have an extra variable indicating that work should be done (this was inherently mutex-protected with the condvar/mutex pair here - other threads needed to lock the mutex before changing it however).
It was technically possible for a thread to return from a condition wait without being kicked by another process (this is a genuine spurious wakeup) but, in all my many years working on pthreads, both in development/service of the code and as a user of them, I never once received one of these. Maybe that was just because HP had a decent implementation :-)
In any case, the same code that handled the erroneous case also handled genuine spurious wakeups as well since the work-available flag would not be set for those.
A condition variable is quite limited if you could only signal a condition, usually you need to handle some data that's related to to condition that was signalled. Signalling/wakeup have to be done atomically in regards to achieve that without introducing race conditions, or be overly complex
pthreads can also give you , for rather technical reasons, a spurious wakeup . That means you need to check a predicate, so you can be sure the condition actually was signalled - and distinguish that from a spurious wakeup. Checking such a condition in regards to waiting for it need to be guarded - so a condition variable needs a way to atomically wait/wake up while locking/unlocking a mutex guarding that condition.
Consider a simple example where you're notified that some data are produced. Maybe another thread made some data that you want, and set a pointer to that data.
Imagine a producer thread giving some data to another consumer thread through a 'some_data'
pointer.
while(1) {
pthread_cond_wait(&cond); //imagine cond_wait did not have a mutex
char *data = some_data;
some_data = NULL;
handle(data);
}
you'd naturally get a lot of race condition, what if the other thread did some_data = new_data right after you got woken up, but before you did data = some_data
You cannot really create your own mutex to guard this case either .e.g
while(1) {
pthread_cond_wait(&cond); //imagine cond_wait did not have a mutex
pthread_mutex_lock(&mutex);
char *data = some_data;
some_data = NULL;
pthread_mutex_unlock(&mutex);
handle(data);
}
Will not work, there's still a chance of a race condition in between waking up and grabbing the mutex. Placing the mutex before the pthread_cond_wait doesn't help you, as you will now
hold the mutex while waiting - i.e. the producer will never be able to grab the mutex.
(note, in this case you could create a second condition variable to signal the producer that you're done with some_data - though this will become complex, especially so if you want many producers/consumers.)
Thus you need a way to atomically release/grab the mutex when waiting/waking up from the condition. That's what pthread condition variables does, and here's what you'd do:
while(1) {
pthread_mutex_lock(&mutex);
while(some_data == NULL) { // predicate to acccount for spurious wakeups,would also
// make it robust if there were several consumers
pthread_cond_wait(&cond,&mutex); //atomically lock/unlock mutex
}
char *data = some_data;
some_data = NULL;
pthread_mutex_unlock(&mutex);
handle(data);
}
(the producer would naturally need to take the same precautions, always guarding 'some_data' with the same mutex, and making sure it doesn't overwrite some_data if some_data is currently != NULL)
POSIX condition variables are stateless. So it is your responsibility to maintain the state. Since the state will be accessed by both threads that wait and threads that tell other threads to stop waiting, it must be protected by a mutex. If you think you can use condition variables without a mutex, then you haven't grasped that condition variables are stateless.
Condition variables are built around a condition. Threads that wait on a condition variable are waiting for some condition. Threads that signal condition variables change that condition. For example, a thread might be waiting for some data to arrive. Some other thread might notice that the data has arrived. "The data has arrived" is the condition.
Here's the classic use of a condition variable, simplified:
while(1)
{
pthread_mutex_lock(&work_mutex);
while (work_queue_empty()) // wait for work
pthread_cond_wait(&work_cv, &work_mutex);
work = get_work_from_queue(); // get work
pthread_mutex_unlock(&work_mutex);
do_work(work); // do that work
}
See how the thread is waiting for work. The work is protected by a mutex. The wait releases the mutex so that another thread can give this thread some work. Here's how it would be signalled:
void AssignWork(WorkItem work)
{
pthread_mutex_lock(&work_mutex);
add_work_to_queue(work); // put work item on queue
pthread_cond_signal(&work_cv); // wake worker thread
pthread_mutex_unlock(&work_mutex);
}
Notice that you need the mutex to protect the work queue. Notice that the condition variable itself has no idea whether there's work or not. That is, a condition variable must be associated with a condition, that condition must be maintained by your code, and since it's shared among threads, it must be protected by a mutex.
Not all condition variable functions require a mutex: only the waiting operations do. The signal and broadcast operations do not require a mutex. A condition variable also is not permanently associated with a specific mutex; the external mutex does not protect the condition variable. If a condition variable has internal state, such as a queue of waiting threads, this must be protected by an internal lock inside the condition variable.
The wait operations bring together a condition variable and a mutex, because:
a thread has locked the mutex, evaluated some expression over shared variables and found it to be false, such that it needs to wait.
the thread must atomically move from owning the mutex, to waiting on the condition.
For this reason, the wait operation takes as arguments both the mutex and condition: so that it can manage the atomic transfer of a thread from owning the mutex to waiting, so that the thread does not fall victim to the lost wake up race condition.
A lost wakeup race condition will occur if a thread gives up a mutex, and then waits on a stateless synchronization object, but in a way which is not atomic: there exists a window of time when the thread no longer has the lock, and has not yet begun waiting on the object. During this window, another thread can come in, make the awaited condition true, signal the stateless synchronization and then disappear. The stateless object doesn't remember that it was signaled (it is stateless). So then the original thread goes to sleep on the stateless synchronization object, and does not wake up, even though the condition it needs has already become true: lost wakeup.
The condition variable wait functions avoid the lost wake up by making sure that the calling thread is registered to reliably catch the wakeup before it gives up the mutex. This would be impossible if the condition variable wait function did not take the mutex as an argument.
I do not find the other answers to be as concise and readable as this page. Normally the waiting code looks something like this:
mutex.lock()
while(!check())
condition.wait(mutex) # atomically unlocks mutex and sleeps. Calls
# mutex.lock() once the thread wakes up.
mutex.unlock()
There are three reasons to wrap the wait() in a mutex:
without a mutex another thread could signal() before the wait() and we'd miss this wake up.
normally check() is dependent on modification from another thread, so you need mutual exclusion on it anyway.
to ensure that the highest priority thread proceeds first (the queue for the mutex allows the scheduler to decide who goes next).
The third point is not always a concern - historical context is linked from the article to this conversation.
Spurious wake-ups are often mentioned with regard to this mechanism (i.e. the waiting thread is awoken without signal() being called). However, such events are handled by the looped check().
Condition variables are associated with a mutex because it is the only way it can avoid the race that it is designed to avoid.
// incorrect usage:
// thread 1:
while (notDone) {
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable
pthread_mutex_unlock(&mutex);
if (ready) {
doWork();
} else {
pthread_cond_wait(&cond1); // invalid syntax: this SHOULD have a mutex
}
}
// signalling thread
// thread 2:
prepareToRunThread1();
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond1);
Now, lets look at a particularly nasty interleaving of these operations
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable;
pthread_mutex_unlock(&mutex);
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond1);
if (ready) {
pthread_cond_wait(&cond1); // uh o!
At this point, there is no thread which is going to signal the condition variable, so thread1 will wait forever, even though the protectedReadyToRunVariable says it's ready to go!
The only way around this is for condition variables to atomically release the mutex while simultaneously starting to wait on the condition variable. This is why the cond_wait function requires a mutex
// correct usage:
// thread 1:
while (notDone) {
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable
if (ready) {
pthread_mutex_unlock(&mutex);
doWork();
} else {
pthread_cond_wait(&mutex, &cond1);
}
}
// signalling thread
// thread 2:
prepareToRunThread1();
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_cond_signal(&mutex, &cond1);
pthread_mutex_unlock(&mutex);
The mutex is supposed to be locked when you call pthread_cond_wait; when you call it it atomically both unlocks the mutex and then blocks on the condition. Once the condition is signaled it atomically locks it again and returns.
This allows the implementation of predictable scheduling if desired, in that the thread that would be doing the signalling can wait until the mutex is released to do its processing and then signal the condition.
It appears to be a specific design decision rather than a conceptual need.
Per the pthreads docs the reason that the mutex was not separated is that there is a significant performance improvement by combining them and they expect that because of common race conditions if you don't use a mutex, it's almost always going to be done anyway.
https://linux.die.net/man/3/pthread_cond_wait
Features of Mutexes and Condition Variables
It had been suggested that the mutex acquisition and release be
decoupled from condition wait. This was rejected because it is the
combined nature of the operation that, in fact, facilitates realtime
implementations. Those implementations can atomically move a
high-priority thread between the condition variable and the mutex in a
manner that is transparent to the caller. This can prevent extra
context switches and provide more deterministic acquisition of a mutex
when the waiting thread is signaled. Thus, fairness and priority
issues can be dealt with directly by the scheduling discipline.
Furthermore, the current condition wait operation matches existing
practice.
There are a tons of exegeses about that, yet I want to epitomize it with an example following.
1 void thr_child() {
2 done = 1;
3 pthread_cond_signal(&c);
4 }
5 void thr_parent() {
6 if (done == 0)
7 pthread_cond_wait(&c);
8 }
What's wrong with the code snippet? Just ponder somewhat before going ahead.
The issue is genuinely subtle. If the parent invokes
thr_parent() and then vets the value of done, it will see that it is 0 and
thus try to go to sleep. But just before it calls wait to go to sleep, the parent
is interrupted between lines of 6-7, and the child runs. The child changes the state variable
done to 1 and signals, but no thread is waiting and thus no thread is
woken. When the parent runs again, it sleeps forever, which is really egregious.
What if they are carried out while acquired locks individually?
I made an exercice in class if you want a real example of condition variable :
#include "stdio.h"
#include "stdlib.h"
#include "pthread.h"
#include "unistd.h"
int compteur = 0;
pthread_cond_t varCond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex_compteur;
void attenteSeuil(arg)
{
pthread_mutex_lock(&mutex_compteur);
while(compteur < 10)
{
printf("Compteur : %d<10 so i am waiting...\n", compteur);
pthread_cond_wait(&varCond, &mutex_compteur);
}
printf("I waited nicely and now the compteur = %d\n", compteur);
pthread_mutex_unlock(&mutex_compteur);
pthread_exit(NULL);
}
void incrementCompteur(arg)
{
while(1)
{
pthread_mutex_lock(&mutex_compteur);
if(compteur == 10)
{
printf("Compteur = 10\n");
pthread_cond_signal(&varCond);
pthread_mutex_unlock(&mutex_compteur);
pthread_exit(NULL);
}
else
{
printf("Compteur ++\n");
compteur++;
}
pthread_mutex_unlock(&mutex_compteur);
}
}
int main(int argc, char const *argv[])
{
int i;
pthread_t threads[2];
pthread_mutex_init(&mutex_compteur, NULL);
pthread_create(&threads[0], NULL, incrementCompteur, NULL);
pthread_create(&threads[1], NULL, attenteSeuil, NULL);
pthread_exit(NULL);
}
On my neverending quest to understand std::contion_variables I've run into the following. On this page it says the following:
void print_id (int id) {
std::unique_lock<std::mutex> lck(mtx);
while (!ready) cv.wait(lck);
// ...
std::cout << "thread " << id << '\n';
}
And after that it says this:
void go() {
std::unique_lock<std::mutex> lck(mtx);
ready = true;
cv.notify_all();
}
Now as I understand it, both of these functions will halt on the std::unqique_lock line. Until a unique lock is acquired. That is, no other thread has a lock.
So say the print_id function is executed first. The unique lock will be aquired and the function will halt on the wait line.
If the go function is then executed (on a separate thread), the code there will halt on the unique lock line. Since the mutex is locked by the print_id function already.
Obviously this wouldn't work if the code was like that. But I really don't see what I'm not getting here. So please enlighten me.
What you're missing is that wait unlocks the mutex and then waits for the signal on cv.
It locks the mutex again before returning.
You could have found this out by clicking on wait on the page where you found the example:
At the moment of blocking the thread, the function automatically calls lck.unlock(), allowing other locked threads to continue.
Once notified (explicitly, by some other thread), the function unblocks and calls lck.lock(), leaving lck in the same state as when the function was called.
There's one point you've missed—calling wait() unlocks the mutex. The thread atomically (releases the mutex + goes to sleep). Then, when woken by the signal, it tries to re-acquire the mutex (possibly blocking); once it acquires it, it can proceed.
Notice that it's not necessary to have the mutex locked for calling notify_*, only for wait*
To answer the question as posed, which seems necessary regarding claims that you should not acquire a lock on notification for performance reasons (isn't correctness more important than performance?): The necessity to lock on "wait" and the recommendation to always lock around "notify" is to protect the user from himself and his program from data and logical races. Without the lock in "go", the program you posted would immediately have a data race on "ready". However, even if ready were itself synchronized (e.g. atomic) you would have a logical race with a missed notification, because without the lock in "go" it is possible for the notify to occur just after the check for "ready" and just before the actual wait, and the waiting thread may then remain blocked indefinitely. The synchronization on the atomic variable itself is not enough to prevent this. This is why helgrind will warn when a notification is done without holding the lock. There are some fringe cases where the mutex lock is really not required around the notify. In all of these cases, there needs to be a bidirectional synchronization beforehand so that the producing thread can know for sure that the other thread is already waiting. IMO these cases are for experts only. Actually, I have seen an expert, giving a talk about multi-threading, getting this wrong — he thought an atomic counter would suffice. That said, the lock around the wait is always necessary for correctness (or, at least, an operation that is atomic with the wait), and this is why the standard library enforces it and atomically unlocks the mutex on entering the wait.
POSIX condition variables are, unlike Windows events, not "idiot-proof" because they are stateless (apart from being aware of waiting threads). The recommendation to use a lock on the notify is there to protect you from the worst and most common screwups. You can build a Windows-like stateful event using a mutex + condition var + bool variable if you like, of course.
I’m reading up on pthread.h; the condition variable related functions (like pthread_cond_wait(3)) require a mutex as an argument. Why? As far as I can tell, I’m going to be creating a mutex just to use as that argument? What is that mutex supposed to do?
It's just the way that condition variables are (or were originally) implemented.
The mutex is used to protect the condition variable itself. That's why you need it locked before you do a wait.
The wait will "atomically" unlock the mutex, allowing others access to the condition variable (for signalling). Then when the condition variable is signalled or broadcast to, one or more of the threads on the waiting list will be woken up and the mutex will be magically locked again for that thread.
You typically see the following operation with condition variables, illustrating how they work. The following example is a worker thread which is given work via a signal to a condition variable.
thread:
initialise.
lock mutex.
while thread not told to stop working:
wait on condvar using mutex.
if work is available to be done:
do the work.
unlock mutex.
clean up.
exit thread.
The work is done within this loop provided that there is some available when the wait returns. When the thread has been flagged to stop doing work (usually by another thread setting the exit condition then kicking the condition variable to wake this thread up), the loop will exit, the mutex will be unlocked and this thread will exit.
The code above is a single-consumer model as the mutex remains locked while the work is being done. For a multi-consumer variation, you can use, as an example:
thread:
initialise.
lock mutex.
while thread not told to stop working:
wait on condvar using mutex.
if work is available to be done:
copy work to thread local storage.
unlock mutex.
do the work.
lock mutex.
unlock mutex.
clean up.
exit thread.
which allows other consumers to receive work while this one is doing work.
The condition variable relieves you of the burden of polling some condition instead allowing another thread to notify you when something needs to happen. Another thread can tell that thread that work is available as follows:
lock mutex.
flag work as available.
signal condition variable.
unlock mutex.
The vast majority of what are often erroneously called spurious wakeups was generally always because multiple threads had been signalled within their pthread_cond_wait call (broadcast), one would return with the mutex, do the work, then re-wait.
Then the second signalled thread could come out when there was no work to be done. So you had to have an extra variable indicating that work should be done (this was inherently mutex-protected with the condvar/mutex pair here - other threads needed to lock the mutex before changing it however).
It was technically possible for a thread to return from a condition wait without being kicked by another process (this is a genuine spurious wakeup) but, in all my many years working on pthreads, both in development/service of the code and as a user of them, I never once received one of these. Maybe that was just because HP had a decent implementation :-)
In any case, the same code that handled the erroneous case also handled genuine spurious wakeups as well since the work-available flag would not be set for those.
A condition variable is quite limited if you could only signal a condition, usually you need to handle some data that's related to to condition that was signalled. Signalling/wakeup have to be done atomically in regards to achieve that without introducing race conditions, or be overly complex
pthreads can also give you , for rather technical reasons, a spurious wakeup . That means you need to check a predicate, so you can be sure the condition actually was signalled - and distinguish that from a spurious wakeup. Checking such a condition in regards to waiting for it need to be guarded - so a condition variable needs a way to atomically wait/wake up while locking/unlocking a mutex guarding that condition.
Consider a simple example where you're notified that some data are produced. Maybe another thread made some data that you want, and set a pointer to that data.
Imagine a producer thread giving some data to another consumer thread through a 'some_data'
pointer.
while(1) {
pthread_cond_wait(&cond); //imagine cond_wait did not have a mutex
char *data = some_data;
some_data = NULL;
handle(data);
}
you'd naturally get a lot of race condition, what if the other thread did some_data = new_data right after you got woken up, but before you did data = some_data
You cannot really create your own mutex to guard this case either .e.g
while(1) {
pthread_cond_wait(&cond); //imagine cond_wait did not have a mutex
pthread_mutex_lock(&mutex);
char *data = some_data;
some_data = NULL;
pthread_mutex_unlock(&mutex);
handle(data);
}
Will not work, there's still a chance of a race condition in between waking up and grabbing the mutex. Placing the mutex before the pthread_cond_wait doesn't help you, as you will now
hold the mutex while waiting - i.e. the producer will never be able to grab the mutex.
(note, in this case you could create a second condition variable to signal the producer that you're done with some_data - though this will become complex, especially so if you want many producers/consumers.)
Thus you need a way to atomically release/grab the mutex when waiting/waking up from the condition. That's what pthread condition variables does, and here's what you'd do:
while(1) {
pthread_mutex_lock(&mutex);
while(some_data == NULL) { // predicate to acccount for spurious wakeups,would also
// make it robust if there were several consumers
pthread_cond_wait(&cond,&mutex); //atomically lock/unlock mutex
}
char *data = some_data;
some_data = NULL;
pthread_mutex_unlock(&mutex);
handle(data);
}
(the producer would naturally need to take the same precautions, always guarding 'some_data' with the same mutex, and making sure it doesn't overwrite some_data if some_data is currently != NULL)
POSIX condition variables are stateless. So it is your responsibility to maintain the state. Since the state will be accessed by both threads that wait and threads that tell other threads to stop waiting, it must be protected by a mutex. If you think you can use condition variables without a mutex, then you haven't grasped that condition variables are stateless.
Condition variables are built around a condition. Threads that wait on a condition variable are waiting for some condition. Threads that signal condition variables change that condition. For example, a thread might be waiting for some data to arrive. Some other thread might notice that the data has arrived. "The data has arrived" is the condition.
Here's the classic use of a condition variable, simplified:
while(1)
{
pthread_mutex_lock(&work_mutex);
while (work_queue_empty()) // wait for work
pthread_cond_wait(&work_cv, &work_mutex);
work = get_work_from_queue(); // get work
pthread_mutex_unlock(&work_mutex);
do_work(work); // do that work
}
See how the thread is waiting for work. The work is protected by a mutex. The wait releases the mutex so that another thread can give this thread some work. Here's how it would be signalled:
void AssignWork(WorkItem work)
{
pthread_mutex_lock(&work_mutex);
add_work_to_queue(work); // put work item on queue
pthread_cond_signal(&work_cv); // wake worker thread
pthread_mutex_unlock(&work_mutex);
}
Notice that you need the mutex to protect the work queue. Notice that the condition variable itself has no idea whether there's work or not. That is, a condition variable must be associated with a condition, that condition must be maintained by your code, and since it's shared among threads, it must be protected by a mutex.
Not all condition variable functions require a mutex: only the waiting operations do. The signal and broadcast operations do not require a mutex. A condition variable also is not permanently associated with a specific mutex; the external mutex does not protect the condition variable. If a condition variable has internal state, such as a queue of waiting threads, this must be protected by an internal lock inside the condition variable.
The wait operations bring together a condition variable and a mutex, because:
a thread has locked the mutex, evaluated some expression over shared variables and found it to be false, such that it needs to wait.
the thread must atomically move from owning the mutex, to waiting on the condition.
For this reason, the wait operation takes as arguments both the mutex and condition: so that it can manage the atomic transfer of a thread from owning the mutex to waiting, so that the thread does not fall victim to the lost wake up race condition.
A lost wakeup race condition will occur if a thread gives up a mutex, and then waits on a stateless synchronization object, but in a way which is not atomic: there exists a window of time when the thread no longer has the lock, and has not yet begun waiting on the object. During this window, another thread can come in, make the awaited condition true, signal the stateless synchronization and then disappear. The stateless object doesn't remember that it was signaled (it is stateless). So then the original thread goes to sleep on the stateless synchronization object, and does not wake up, even though the condition it needs has already become true: lost wakeup.
The condition variable wait functions avoid the lost wake up by making sure that the calling thread is registered to reliably catch the wakeup before it gives up the mutex. This would be impossible if the condition variable wait function did not take the mutex as an argument.
I do not find the other answers to be as concise and readable as this page. Normally the waiting code looks something like this:
mutex.lock()
while(!check())
condition.wait(mutex) # atomically unlocks mutex and sleeps. Calls
# mutex.lock() once the thread wakes up.
mutex.unlock()
There are three reasons to wrap the wait() in a mutex:
without a mutex another thread could signal() before the wait() and we'd miss this wake up.
normally check() is dependent on modification from another thread, so you need mutual exclusion on it anyway.
to ensure that the highest priority thread proceeds first (the queue for the mutex allows the scheduler to decide who goes next).
The third point is not always a concern - historical context is linked from the article to this conversation.
Spurious wake-ups are often mentioned with regard to this mechanism (i.e. the waiting thread is awoken without signal() being called). However, such events are handled by the looped check().
Condition variables are associated with a mutex because it is the only way it can avoid the race that it is designed to avoid.
// incorrect usage:
// thread 1:
while (notDone) {
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable
pthread_mutex_unlock(&mutex);
if (ready) {
doWork();
} else {
pthread_cond_wait(&cond1); // invalid syntax: this SHOULD have a mutex
}
}
// signalling thread
// thread 2:
prepareToRunThread1();
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond1);
Now, lets look at a particularly nasty interleaving of these operations
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable;
pthread_mutex_unlock(&mutex);
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond1);
if (ready) {
pthread_cond_wait(&cond1); // uh o!
At this point, there is no thread which is going to signal the condition variable, so thread1 will wait forever, even though the protectedReadyToRunVariable says it's ready to go!
The only way around this is for condition variables to atomically release the mutex while simultaneously starting to wait on the condition variable. This is why the cond_wait function requires a mutex
// correct usage:
// thread 1:
while (notDone) {
pthread_mutex_lock(&mutex);
bool ready = protectedReadyToRunVariable
if (ready) {
pthread_mutex_unlock(&mutex);
doWork();
} else {
pthread_cond_wait(&mutex, &cond1);
}
}
// signalling thread
// thread 2:
prepareToRunThread1();
pthread_mutex_lock(&mutex);
protectedReadyToRuNVariable = true;
pthread_cond_signal(&mutex, &cond1);
pthread_mutex_unlock(&mutex);
The mutex is supposed to be locked when you call pthread_cond_wait; when you call it it atomically both unlocks the mutex and then blocks on the condition. Once the condition is signaled it atomically locks it again and returns.
This allows the implementation of predictable scheduling if desired, in that the thread that would be doing the signalling can wait until the mutex is released to do its processing and then signal the condition.
It appears to be a specific design decision rather than a conceptual need.
Per the pthreads docs the reason that the mutex was not separated is that there is a significant performance improvement by combining them and they expect that because of common race conditions if you don't use a mutex, it's almost always going to be done anyway.
https://linux.die.net/man/3/pthread_cond_wait
Features of Mutexes and Condition Variables
It had been suggested that the mutex acquisition and release be
decoupled from condition wait. This was rejected because it is the
combined nature of the operation that, in fact, facilitates realtime
implementations. Those implementations can atomically move a
high-priority thread between the condition variable and the mutex in a
manner that is transparent to the caller. This can prevent extra
context switches and provide more deterministic acquisition of a mutex
when the waiting thread is signaled. Thus, fairness and priority
issues can be dealt with directly by the scheduling discipline.
Furthermore, the current condition wait operation matches existing
practice.
There are a tons of exegeses about that, yet I want to epitomize it with an example following.
1 void thr_child() {
2 done = 1;
3 pthread_cond_signal(&c);
4 }
5 void thr_parent() {
6 if (done == 0)
7 pthread_cond_wait(&c);
8 }
What's wrong with the code snippet? Just ponder somewhat before going ahead.
The issue is genuinely subtle. If the parent invokes
thr_parent() and then vets the value of done, it will see that it is 0 and
thus try to go to sleep. But just before it calls wait to go to sleep, the parent
is interrupted between lines of 6-7, and the child runs. The child changes the state variable
done to 1 and signals, but no thread is waiting and thus no thread is
woken. When the parent runs again, it sleeps forever, which is really egregious.
What if they are carried out while acquired locks individually?
I made an exercice in class if you want a real example of condition variable :
#include "stdio.h"
#include "stdlib.h"
#include "pthread.h"
#include "unistd.h"
int compteur = 0;
pthread_cond_t varCond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mutex_compteur;
void attenteSeuil(arg)
{
pthread_mutex_lock(&mutex_compteur);
while(compteur < 10)
{
printf("Compteur : %d<10 so i am waiting...\n", compteur);
pthread_cond_wait(&varCond, &mutex_compteur);
}
printf("I waited nicely and now the compteur = %d\n", compteur);
pthread_mutex_unlock(&mutex_compteur);
pthread_exit(NULL);
}
void incrementCompteur(arg)
{
while(1)
{
pthread_mutex_lock(&mutex_compteur);
if(compteur == 10)
{
printf("Compteur = 10\n");
pthread_cond_signal(&varCond);
pthread_mutex_unlock(&mutex_compteur);
pthread_exit(NULL);
}
else
{
printf("Compteur ++\n");
compteur++;
}
pthread_mutex_unlock(&mutex_compteur);
}
}
int main(int argc, char const *argv[])
{
int i;
pthread_t threads[2];
pthread_mutex_init(&mutex_compteur, NULL);
pthread_create(&threads[0], NULL, incrementCompteur, NULL);
pthread_create(&threads[1], NULL, attenteSeuil, NULL);
pthread_exit(NULL);
}
I have been carefully studying the accepted answer to the following SO question: C++0x has no semaphores? How to synchronize threads?
In the semaphore implementation in that answer, here is the implementation of the wait() function:
void wait()
{
boost::mutex::scoped_lock lock(mutex_);
while(!count_)
condition_.wait(lock);
--count_;
}
I am trying to understand the purpose of the while(!count_) condition.
The answer to another SO question ( How does this implementation of semaphore work? ) indicates that when notify_one() is called on the condition variable, that it is possible that MORE THAN ONE thread waiting on that condition variable will be woken up - hence the need for the while loop. I would like to have this confirmed - is that the full and/or correct answer, or are there other reasons why the while loop is necessary?
If more than one thread wakes up, which thread owns the mutex? The more I think about it, the more ill-defined it seems to be if more than one thread can wake up due to a single call to notify_one(). Would it not be possible for BOTH woken-up threads to see the count_ value as higher than 0, and proceed to both decrement count_, resulting in a count_ value of less than 0, and defeating the purpose (and correctness) of the semaphore?
There could be spurious wakeups, or notify_one could wake up more than one thread due to implementation details, as you've already mentioned.
Waking up multiple threads doesn't mean though that all of them can enter the protected section at the same time, it just means that when ThreadA releases the lock, ThreadB (which got woken up along with ThreadA in the previous example) also gets to enter the protected section. By this time ThreadA has already done its work, so ThreadB won't see the count variable in the same state as ThreadA found it.