Re: [boost] [signals/threadsafe version] Atomic disconnects

From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2008-03-03 13:08:01

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On Monday 11 February 2008 10:11 am, Johan Torp wrote:
> I'm wondering if the new thread safe implementation of signals guarantees
> atomic disconnects.
>
> line1: signal<void(), multi_threaded> sig;
> line2: thread t(sig); // Start a new thread which will emit "sig"
> line3: sig.connect(&some_func);
> line4: sig.disconnect(&some_func);
> line5: ...
>
> Am I guaranteed that some_func isn't called at line 5 or afterwards?

Sorry it took me so long to reply to this, I just noticed it.  If some_func is 
in the process of running in another thread when you call disconnect(), it 
may still be running when disconnect() returns.  disconnect() does not block 
waiting for any slots to complete.  In fact, no mutexes inside the signal are 
held while a slot is executing, to avoid deadlock issues.

> If disconnect is blocking, how do you prevent the following dead-lock
> during signal emission?
> - Emitting thread tries to acquire a mutex X via a slot
> - Disconnect calling thread is holding X and gets blocked on disconnect

- -- 
Frank
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