[Qt-interest] QMutex deadlock without using QMutex

[Andre Somers]

Hi Gabriel,

Thanks for your insights.

On 27-4-2010 18:10, Gabriel M. Beddingfield wrote:
> i.e. you're relying Qt's Queued Connections for thread synchronization 
> primatives.  These, of course, are using Mutexes and Sempahores and 
> Atomic ops.
That's why I said I'm not using them explicitly. Meaning that perhaps Qt 
uses them behind the scenes anyway, as it turns out it is.
>
>> 0    QMutex::lock    qmutex.cpp    201    0x00e10f4e
>> 1    QMutexLocker::relock    qmutex.h    120    0x00f5c564
>> 2    QMutexLocker    qmutex.h    102    0x00f5c604
>> 3    QCoreApplicationPrivate::sendPostedEvents
>> qcoreapplication.cpp    1271    0x00efe127
>> 4    qt_internal_proc    qeventdispatcher_win.cpp    482    0x00f1f73f
>> 5    USER32!DefDlgProcW    C:\WINDOWS\syswow64\user32.dll    0
>> 0x7d9472d8
>> 6    ??        0    0x003f06a8
>> 7    ??        0    0x00000401
>> 8    ??        0    0x00000000
>>
>> Following the item at line 3, I notice that Qt is indeed using a mutex
>> for the message passing. So, it seems we are dealing with Qt's mutex,
>> not mine.
>>
>> Question is then: why does Qt's message system get into a deadlock?
>
> Not sure.  Deadlocks are hard to solve.
>
> In case you don't know, it usually happens when this is effectively 
> executed:
>
>     QMutex mut;
>
>     void some_func()
>     {
>         mut.lock();
>         //...
>         mut.lock(); // <== deadlock
>     }
>
> The second lock() will wait for the mutex to unlock, which will never 
> happen.
>
> Perhaps switch from Qt::QueuedConnection to 
> Qt::BlockingQueuedConnection (or the reverse).  You might get lucky.
Ehh? Swithing to a blocking connection would defeat the purpose of using 
threading in the first place I think? If the work manager thread has to 
wait for each worker thread to finish it's work before being able to 
handle anything else, I would effectively have a single threaded 
application as the other threads do nothing but wait for work, I 
think... Or do I totally misunderstand the documentation here? If I try 
it, I get up to about 25% CPU usage on my quad core system. That's 
single threaded performance.

André