Re: a recursive lock prototype

[Marcus Brinkmann]

On Thu, Jul 05, 2001 at 08:57:15AM -0700, Thomas Bushnell, BSG wrote:
> Marcus Brinkmann <brinkmd@master.debian.org> writes:
> 
> > We agreed, in an inspiring coordinated coding effort, on the following
> > implementation (thanks to Johannes, Neal, Moshe and Rene):
> 
> No, that still has a race condition as follows.  We begin in the fully
> unlocked state.
> 
> THREAD 1                        THREAD 2
> 
> Start recursive_lock
> Read RL->locking_thread
>                 Context Switch
>                                 Start recursive_lock, function finishes       
>  
>                 Context Switch
> Check RL->locking_thread
>   it seems to match, and the lock
>   incorrectly succeeds.
> 
> You need some kind of guard around this structure!

How can the check after the second context switch succeed?
Thread 2 will change the value from MACH_PORT_NULL to the port name
of it's thread port, neither matches the port name of thread 1.
So the evaluation of the if condition is a constant
(rl->locking_thread == thread)

I found it really weird that no guard is needed, but actually refcount
is guarded by the mutex, as are all writers of the locking_thread.
The only unguarded reader of that is the above if, and we considered 
all cases we could think of and came to the conclusion that it is robust.

Thanks,
Marcus