Does anyone know how to check and see if a QMutex is locked, without using the function:
bool QMutex::tryLock()
The reason I don't want to use tryLock() is because it does two things:
Check and see if the mutex is locked.
If it's not locked then lock it.
For my purposes, I am not interested in performing this second step (locking the mutex).
I just want to know if it is locked or not.
Trying to lock a mutex is by definition the only way to tell if it's locked; otherwise when this imaginary function returned, how would you know if the mutex was still locked? It may have become unlocked while the function was returning; or more importantly, without performing all the cache-flushing and syncronization necessary to lock it, you couldn't actually be sure if it was locked or not.
OK, I'm guessing there is no real way to do what I'm asking without actually using tryLock().
This could be accomplished with the following code:
bool is_locked = true;
if( a_mutex.tryLock() )
{
a_mutex.unlock();
is_locked = false;
}
if( is_locked )
{
...
}
As you can see, it unlocks the QMutex, "a_mutex", if it was able to lock it.
Of course, this is not a perfect solution, as by the time it hits the 2nd if statement, the mutex's status could have changed.
Maybe a QSemaphore with one permit? The available() method may give you what you need.
QMutex is designed just for locking and unlocking functionality. Gathering statistics may be satisfied with some custom counters.
Try QSemaphore as #Luca Carion mentioned earlier.
static bool isLocked(const QBasicMutex *mut) {
auto mdata = reinterpret_cast<const QBasicAtomicPointer<QMutexData> *>(mut);
return mdata->load();
}
This code should work on Qt 5 and doesn't mess with the mutex state.
Every QBasicMutex holds a single (atomic) pointer (called d_ptr) that is NULL if not owned, a special value if it is owned but uncontested or a pointer to a platform dependent structure (on Unix, this is basically a pthread mutex) if the mutex is owned and contested.
We need the reinterpret_cast because d_ptr is private.
More info can be found here: https://woboq.com/blog/internals-of-qmutex-in-qt5.html
A legitimate use case is to verify that a mutex is indeed locked, for example if it is a function precondition. I suggest using Q_ASSERT(isLocked(...)) for this purpose.
Testing for an unlocked mutex is inherently unsafe and should not be done.
Related
I have a situation where I have multiple threads accessing the clipboard and performing an action with some sleeps. Basically I don't want to copy the wrong thing to the clipboard and perform something in an incorrect fashion.
Currently, I have an idea of using a mutex to lock a std::string variable and store the required data in it, then to the clipboard. After I finish the action, I unlock it. Then the other thread accesses the variable, then clipboard, and action.
As I have never used a mutex, my question is - will it work by just doing the above?
I'm using the following library to manage the clipboard: https://github.com/dacap/clip
From a quick look at the library, you may not need to use a mutex at all. For eg. the set_text function:
bool set_text(const std::string& value) {
lock l;
if (l.locked()) {
l.clear();
return l.set_data(text_format(), value.c_str(), value.size());
}
else
return false;
}
You can see that a lock is being created (which locks the resource based on the os implementation at construction of lock l) which should provide you some thread safety.
Currently, I have an idea of using a mutex to lock a std::string variable and store the required data in it, then to the clipboard.
From the way you explain this it is clear you do not understand how it works. Mutex does not lock a variable whatever type it has. Mutex is locked itself. And mutex has a property that only one thread can lock it at a time. Using that property you can organize your program the way so only one thread will access some critical data and that will make such access thread safe. It is not that you link a mutex to an unrelated variable, then that mutex would somehow lock that variable and make it thread safe. Now if you understand what is different btw those you would understand that yes you can make access to your std::string variable thread safe but that will require all threads accessing it doing that under mutex lock, not that only one thread locks it and all other will obey that lock magically.
I understand how to use condition variables (crummy name for this construct, IMO, as the cv object neither is a variable nor indicates a condition). So I have a pair of threads, canonically set up with Boost.Thread as:
bool awake = false;
boost::mutex sync;
boost::condition_variable cv;
void thread1()
{
boost::unique_lock<boost::mutex> lock1(sync);
while (!awake)
cv.wait(lock1);
lock1.unlock(); // this line actually not canonical, but why not?
// proceed...
}
void thread2()
{
//...
boost::unique_lock<boost::mutex> lock2;
awake = true;
lock2.unlock();
cv.notify_all();
}
My question is: does thread2 really need to be protecting the assignment to awake? It seems to me the notify_all() call should be sufficient. If the data being manipulated and checked against were more than a simple "ok to proceed" flag, I see the value in the mutex, but here it seems like overkill.
A secondary question is that asked in the code fragment: Why doesn't the Boost documentation show the lock in thread1 being unlocked before the "process data" step?
EDIT: Maybe my question is really: Is there a cleaner construct than a CV to implement this kind of wait?
does thread2 really need to be protecting the assignment to awake?
Yes. Modifying an object from one thread and accessing it from another without synchronisation gives undefined behaviour. Even if it's just a bool.
For example, on some multiprocessor systems the write might only affect local memory; without an explicit synchronisation operation, other threads might never see the change.
Why doesn't the Boost documentation show the lock in thread1 being unlocked before the "process data" step?
If you unlocked the mutex before clearing the flag, then you might miss another signal.
Is there a cleaner construct than a CV to implement this kind of wait?
In Boost and the standard C++ library, no; a condition variable is flexible enough to handle arbitrary shared state and not particularly over-complicated for this simple case, so there's no particular need for anything simpler.
More generally, you could use a semaphore or a pipe to send a simple signal between threads.
Formally, you definitely need the lock in both threads: if any thread
modifies an object, and more than one thread accesses it, then all
accesses must be synchronized.
In practice, you'll probably get away with it without the lock; it's
almost certain that notify_all will issue the necessary fence or
membar instructions to ensure that the memory is properly synchronized.
But why take the risk?
As to the absense of the unlock, that's the whole point of the scoped
locking pattern: the unlock is in the destructor of the object, so
that the mutex will be unlocked even if an exception passes through.
I have am reviewing a colleague's Visual Studio 2008 C++03 application application and I've come across an implementation of a thread synchronization primitive (below).
Assuming SyncObject is implemented correctly, is the use of a boolean in the below code to know if the resource is locked or unlocked thread-safe? If no, can you walk through a "ThreadA" does this and "ThreadB" does that situation so I understand your logic?
class CMyLock
{
public:
CMyLock(SyncObject* object)
: object_(object), acquired_(false)
{
};
// return true if the resource is locked within the given timeout period
bool Lock(DWORD dwTimeOut = INFINITE)
{
acquired_ = object_->Lock(dwTimeOut);
return acquired_;
};
// return true if the resource is unlocked
bool Unlock()
{
if (acquired_)
acquired_ = !object_->Unlock();
return !acquired_;
};
// return true if the resource is locked
bool IsLocked() { return acquired_; };
private:
bool acquired_;
// some thread synchronization primitive
SyncObject* object_;
};
It is not threadsafe.
directly after m_pObject->Unlock() returns, another thread waiting on m_pObject->Lock(dwTimeOut) can return and set m_bAcquired to true, then the unlocking thread sets m_bAcquired to false and overwrites the locked state incorrectly (IsLocked will return false while the object is locked).
I see several serious problems with this code. In a code review, I would reject it.
It is not clear what this class is intended to be. It might be a thin proxy to a primitive. It might be an automatic locker. In either case, the design is wrong, and the documentation (none) does not elaborate.
It does not use RAII. This is a good idea no matter what this object is intended to be, but in the case of an automatic locker it is especially important.
It retains it's own state, which could be out of sync with other instances in the same thread. If, for example, you create 2 instances of this object on thread A, set one to locked and check the state of the other one, it should say locked but it won't.
Possibly most importantly, it is re-inventing the wheel at best.
Short answer: no
You need to lock for reading too, or you risk seeing a stale state.
no it's not - at least from what I can see. What may happen is that one thread calls lock and gets the lock and another thread accesses m_bAcquired before it's updated by the thread the causes the lock.
That's why you need a lock for reading as well asl Matthieu M. stated.
A: Lock
after m_pObject is locked but before m_bAcquired is set
B: IsLocked
--> returns false
A - still in Lock: m_pObject = true
so B has false information.
Other problem:
Unlock relies on m_bAcquired.
I think this object is meant to be used from within one single thread. So each thread has its own CSingleLock instance but they all use the same SyncObject. In this case, only SyncObject needs to be thread safe and it works.
I'm using the C++ boost::thread library, which in my case means I'm using pthreads. Officially, a mutex must be unlocked from the same thread which locks it, and I want the effect of being able to lock in one thread and then unlock in another. There are many ways to accomplish this. One possibility would be to write a new mutex class which allows this behavior.
For example:
class inter_thread_mutex{
bool locked;
boost::mutex mx;
boost::condition_variable cv;
public:
void lock(){
boost::unique_lock<boost::mutex> lck(mx);
while(locked) cv.wait(lck);
locked=true;
}
void unlock(){
{
boost::lock_guard<boost::mutex> lck(mx);
if(!locked) error();
locked=false;
}
cv.notify_one();
}
// bool try_lock(); void error(); etc.
}
I should point out that the above code doesn't guarantee FIFO access, since if one thread calls lock() while another calls unlock(), this first thread may acquire the lock ahead of other threads which are waiting. (Come to think of it, the boost::thread documentation doesn't appear to make any explicit scheduling guarantees for either mutexes or condition variables). But let's just ignore that (and any other bugs) for now.
My question is, if I decide to go this route, would I be able to use such a mutex as a model for the boost Lockable concept. For example, would anything go wrong if I use a boost::unique_lock< inter_thread_mutex > for RAII-style access, and then pass this lock to boost::condition_variable_any.wait(), etc.
On one hand I don't see why not. On the other hand, "I don't see why not" is usually a very bad way of determining whether something will work.
The reason I ask is that if it turns out that I have to write wrapper classes for RAII locks and condition variables and whatever else, then I'd rather just find some other way to achieve the same effect.
EDIT:
The kind of behavior I want is basically as follows. I have an object, and it needs to be locked whenever it is modified. I want to lock the object from one thread, and do some work on it. Then I want to keep the object locked while I tell another worker thread to complete the work. So the first thread can go on and do something else while the worker thread finishes up. When the worker thread gets done, it unlocks the mutex.
And I want the transition to be seemless so nobody else can get the mutex lock in between when thread 1 starts the work and thread 2 completes it.
Something like inter_thread_mutex seems like it would work, and it would also allow the program to interact with it as if it were an ordinary mutex. So it seems like a clean solution. If there's a better solution, I'd be happy to hear that also.
EDIT AGAIN:
The reason I need locks to begin with is that there are multiple master threads, and the locks are there to prevent them from accessing shared objects concurrently in invalid ways.
So the code already uses loop-level lock-free sequencing of operations at the master thread level. Also, in the original implementation, there were no worker threads, and the mutexes were ordinary kosher mutexes.
The inter_thread_thingy came up as an optimization, primarily to improve response time. In many cases, it was sufficient to guarantee that the "first part" of operation A, occurs before the "first part" of operation B. As a dumb example, say I punch object 1 and give it a black eye. Then I tell object 1 to change it's internal structure to reflect all the tissue damage. I don't want to wait around for the tissue damage before I move on to punch object 2. However, I do want the tissue damage to occur as part of the same operation; for example, in the interim, I don't want any other thread to reconfigure the object in such a way that would make tissue damage an invalid operation. (yes, this example is imperfect in many ways, and no I'm not working on a game)
So we made the change to a model where ownership of an object can be passed to a worker thread to complete an operation, and it actually works quite nicely; each master thread is able to get a lot more operations done because it doesn't need to wait for them all to complete. And, since the event sequencing at the master thread level is still loop-based, it is easy to write high-level master-thread operations, as they can be based on the assumption that an operation is complete (more precisely, the critical "first part" upon which the sequencing logic depends is complete) when the corresponding function call returns.
Finally, I thought it would be nice to use inter_thread mutex/semaphore thingies using RAII with boost locks to encapsulate the necessary synchronization that is required to make the whole thing work.
man pthread_unlock (this is on OS X, similar wording on Linux) has the answer:
NAME
pthread_mutex_unlock -- unlock a mutex
SYNOPSIS
#include <pthread.h>
int
pthread_mutex_unlock(pthread_mutex_t *mutex);
DESCRIPTION
If the current thread holds the lock on mutex, then the
pthread_mutex_unlock() function unlocks mutex.
Calling pthread_mutex_unlock() with a mutex that the
calling thread does not hold will result in
undefined behavior.
...
My counter-question would be - what kind of synchronization problem are you trying to solve with this? Most probably there is an easier solution.
Neither pthreads nor boost::thread (built on top of it) guarantee any order in which a contended mutex is acquired by competing threads.
Sorry, but I don't understand. what will be the state of your mutex in line [1] in the following code if another thread can unlock it?
inter_thread_mutex m;
{
m.lock();
// [1]
m.unlock();
}
This has no sens.
There's a few ways to approach this. Both of the ones I'm going to suggest are going to involve adding an additional piece of information to the object, rather adding a mechanism to unlock a thread from a thread other than the one that owns it.
1) you can add some information to indicate the object's state:
enum modification_state { consistent, // ready to be examined or to start being modified
phase1_complete, // ready for the second thread to finish the work
};
// first worker thread
lock();
do_init_work(object);
object.mod_state = phase1_complete;
unlock();
signal();
do_other_stuff();
// second worker thread
lock()
while( object.mod_state != phase1_complete )
wait()
do_final_work(obj)
object.mod_state = consistent;
unlock()
signal()
// some other thread that needs to read the data
lock()
while( object.mod_state != consistent )
wait();
read_data(obj)
unlock()
Works just fine with condition variables, because obviously you're not writing your own lock.
2) If you have a specific thread in mind, you can give the object an owner.
// first worker
lock();
while( obj.owner != this_thread() ) wait();
do_initial_work(obj);
obj.owner = second_thread_id;
unlock()
signal()
...
This is pretty much the same solution as my first solution, but more flexible in the adding/removing of phases, and less flexible in the adding/removing of threads.
To be honest, I'm not sure how inter thread mutex would help you here. You'd still need a semaphore or condition variable to signal the passing of the work to the second thread.
Small modification to what you already have: how about storing the id of the thread which you want to take the lock, in your inter_thread_whatever? Then unlock it, and send a message to that thread, saying "I want you execute whatever routine it is that tries to take this lock".
Then the condition in lock becomes while(locked || (desired_locker != thisthread && desired_locker != 0)). Technically you've "released the lock" in the first thread, and "taken it again" in the second thread, but there's no way that any other thread can grab it in between, so it's as if you've transferred it directly from one to the other.
There's a potential problem, that if a thread exits or is killed, while it's the desired locker of your lock, then that thread deadlocks. But you were already talking about the first thread waiting for a message from the second thread to say that it has successfully acquired the lock, so presumably you already have a plan in mind for what happens if that message is never received. To that plan, add "reset the desired_locker field on the inter_thread_whatever".
This is all very hairy, though, I'm not convinced that what I've proposed is correct. Is there a way that the "master" thread (the one that's directing all these helpers) can just make sure that it doesn't order any more operations to be performed on whatever is protected by this lock, until the first op is completed (or fails and some RAII thing notifies you)? You don't need locks as such, if you can deal with it at the level of the message loop.
I don't think it is a good idea to say that your inter_thread_mutex (binary_semaphore) can be seen as a model of Lockable. The main issue is that the main feature of your inter_thread_mutex defeats the Locakble concept. If inter_thread_mutex was a model of lockable you will expect in In [1] that the inter_thread_mutex m is locked.
// thread T1
inter_thread_mutex m;
{
unique_lock<inter_thread_mutex> lk(m);
// [1]
}
But as an other thread T2 can do m.unlock() while T1 is in [1], the guaranty is broken.
Binary semaphores can be used as Lockables as far as each thread tries to lock before unlocking. But the main goal of your class is exactly the contrary.
This is one of the reason semaphores in Boost.Interprocess don't use lock/unlock to name the functions, but wait/notify. Curiously these are the same names used by conditions :)
A mutex is a mechanism for describing mutually exclusive blocks of code. It does not make sense for these blocks of code to cross thread boundaries. Trying to use such a concept in such an counter intuitive way can only lead to problems down the line.
It sounds very much like you're looking for a different multi-threading concept, but without more detail it's hard to know what.
I'm using a QReadWriteLock in my application to guard access to a resource object.
I'm using QReadLocks and QWriteLocks where I can, but sometimes I need a "long-lived" lock that crosses function boundaries. So sometimes I need to be able to query the state of the QReadWriteLock (or QMutex, same thing in this situation) because unlocking a mutex that is already unlocked is very bad.
For QMutex:
Unlocking a mutex that is not locked results in undefined behavior.
For QReadWriteLock:
Attempting to unlock a lock that is not locked is an error, and will result in program termination.
Being able to query the state of the mutex would also come in very handy for debug assertions to test that before starting some operation, I really do have that "long-lived" lock.
But all I really want is to be able to unlock a mutex if I have the lock, and not unlock it if I don't have it. That would make my day. IMO the unlock() function should already do this for me, but it doesn't.
Any way around this?
(and no, using Recursive mode does not help in this situation)
Untested code,
But this should be possible to do, if Qt Doc is correct (though it's a little short about when tryLock() returns true/false)
QMutex m;
bool queryMutexState(){
//static QMutex lock; if you call this frequent, you may need this "protector"
lock.lock();
bool b(m.tryLock());
if (b)
m.unlock();
//lock.unlock()
return b;
}
Try to lock, if it fails, return false (=mutex is aquired somewhere else), if it is not locked, tryLock() will lock it, so unlock it again and return true (=mutex is availiable)
Note:
Another option would be a custom class with member QMutex and bool to indicate lock state. This may work better for you, as the mutex does not need to be locked and unlocked to get the state.
I suggest you go for method #2
Use QMutex::locked() to determine the state of the mutex.
I don't believe there's an equivalent function in QReadWriteLock. That being said, you could use the tryLockForRead(), tryLockForWrite() functions (which return true or false depending) to ensure you managed to get the locks, and if so unlock them later.