Subj : Re: recursive mutexes
To   : comp.programming.threads
From : Uenal Mutlu
Date : Wed May 18 2005 10:12 pm

"doug" wrote>
> "Uenal Mutlu" wrote in message
> > "Torsten Robitzki" wrote
> >> Uenal Mutlu wrote:
> >>
> >> > "doug" wrote
> >> >
> >> >>>You seem to not understand what I'm talking about.
> >> >>>Do you understand this fact: "Recursive locking is defined for the
> >> >>>current
> >> >>>lock owning thread only"?
> >> >>>Since you already have the lock you can do what ever you want since
> >> >>>nobody
> >> >>>else can change anything, but you can do! You simply increment it in
> >> >>>Lock()
> >> >>>and decrement it in Unlock(). Because it is safe because you already
> >> >>>have
> >> >>>it locked.
> >> >>>
> >> >>
> >> >>Think carefully.  How is this possible?
> >> >
> >> > It is really basic stuff. Ask yourself how you would extend a
> >> > non-recursive
> >> > locking method to make it recursive? Recursivity starts with the 2nd
> >> > Lock()
> >> > call on the same object within the same thread, true?
> >>
> >> First one have to check if this lock request comes from the thread
> >> holding the mutex or not. This can not be implemented by a simple
> >> increment.
> >
> > I can only repeat: The code which increments the counter cannot be
> > executed by anyone else but the current lock holding thread.
> > Do you know what a critical section is?
>
> He's getting at this - how do you decide if you own the lock?  If multiple
> threads can possibly be reading/writing the same location, you need
> synchronisation.  If not, why are you even using a lock in the first place?
>
> You say "The code which increments the counter cannot be executed by anyone
> else but the current lock holding thread."  But how does the current thread
> know if has the lock?  I assume this is code internal to the Lock() routine,
> but surely it must be accessing some shared data - a field in the lock
> saying who owns it, for example?
>
> See my post above.  Write some pseudocode for Lock() and we'll see.

For the code see my other reply to you.

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