Review Request 121134: make KGlobal reference counting threadsafe

[Thomas Lübking]

> On Nov. 16, 2014, 7:22 nachm., Thomas Lübking wrote:
> > kdecore/kernel/kglobal.cpp, line 343
> > <https://git.reviewboard.kde.org/r/121134/diff/5/?file=328836#file328836line343>
> >
> >     Does this actually make sense?
> >     The result is "undefined" when used from multiple threads anyway.
> >     
> >     Should one enforce this to be called from the main thread only instead?
> 
> René J.V. Bertin wrote:
>     How do you mean, undefined? The result will be the value set by whoever called the function last.
>     
>     You have a point that this doesn't make a lot of sense, there's something to say that the main (master...) thread ought to be the one deciding whether or not it can be quit by the reference counter.

> How do you mean, undefined?

It's a write-only flag. If written from two threads with disagree, none has an idea about the current state - and there's no getter either.

Since the client code somehow has to find agreement on the outcome, it needs a local mutex - or define who's in position to write this ("the one deciding whether or not it can be quit by the reference counter")


- Thomas


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On Nov. 16, 2014, 8:22 nachm., René J.V. Bertin wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://git.reviewboard.kde.org/r/121134/
> -----------------------------------------------------------
> 
> (Updated Nov. 16, 2014, 8:22 nachm.)
> 
> 
> Review request for KDE Software on Mac OS X, kdelibs and David Faure.
> 
> 
> Repository: kdelibs
> 
> 
> Description
> -------
> 
> I have been experiencing unexpected exits from KDevelop that were not due to any kind of error in the KDevelop code; it was as if someone told the application to exit normally. This happens mostly on OS X, but also sometimes on Linux.
> I finally traced this to `KGlobal::deref` reaching a zero reference count and invoking `QCoreApplication::quit` when called from one of KDevelop's KJob dtors. There does not appear to be a reference counting mismatch in the code, so the issue might be due to a race condition in KGlobal::ref/deref.
> 
> This patch introduces thread-safety to KGlobal's reference counting by turning the simple global `static int s_refCount` into a `static QAtomicInt s_refCount`. I consider this an important bug fix regardless of whether it corrects the issue I have with KDevelop.
> 
> 
> Diffs
> -----
> 
>   kdecore/kernel/kglobal.cpp cf003a4 
> 
> Diff: https://git.reviewboard.kde.org/r/121134/diff/
> 
> 
> Testing
> -------
> 
> On OS X 10.6.8 only for now.
> 
> NB: Qt itself uses the QAtomicInt's parent class (QBasicAtomicInt) for its own reference counter used for implicit sharing. This is thus well-tested code that is unlikely to introduce regressions to a core KDE feature.
> 
> 
> Thanks,
> 
> René J.V. Bertin
> 
>

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