Subj : Re: recursive mutexes
To   : comp.programming.threads
From : doug
Date : Mon May 16 2005 09:16 am


> 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 possible if you store 'have I got this lock?' in thread local storage. 
So yes, by keeping a counter here, you don't need a memory barrier in 
recursive Lock()/Unlock() *.  However, when you find that you don't have the 
semaphore, you still need synchronisation between threads so that the sema4 
is acquired safely.
What you've ended up doing here is accessing TLS for *every* acquire - thus 
making the common case of 'semaphore not acquired' slower.

(* not true on certain SMP architectures.  Without a memory barrier in 
Lock()/Unlock(), a thread rescheduled on a different CPU may mis-read the 
recursive counter (depending on how TLS is implemented).)

Time to read your next chapter...

.