Subj : Re: recursive mutexes
To   : comp.programming.threads
From : doug
Date : Mon May 16 2005 03:55 pm


"Uenal Mutlu" <520001085531-0001@t-online.de> wrote in message 
news:d6aae5$9ac$02$1@news.t-online.com...
> "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.


>
> In your other posting you wrote:
>
>> P.S. never used recursive locks and never had any serious problems with
>> deadlocks.
>
> May I ask: have you ever either implemented any mutex (recursive or 
> non-recursive)
> by yourself or have you ever used a non-recursive mutex in your projects?
> For which OS and using which threading pkg do you develop?
>
>
>

.