Subj : Re: condvar wait / signal on arbitrary object
To   : comp.programming.threads
From : Chris Thomasson
Date : Tue Jul 12 2005 11:47 pm

> Or (I'm not certain that this actually makes sense but ...) each condvar
> could have its own mutex.
>
>   struct ctx {
>       pthread_cond_t condvars[CTX_ARRAY_SIZE];
>       pthread_mutex_t mutexes[CTX_ARRAY_SIZE];
>   };
>   ...
>   int
>   ctx_pthread_cond_wait(struct ctx *ctx, void *obj)
>   {
>       int idx = (size_t)obj % CTX_ARRAY_SIZE;
>       pthread_cond_t *cond = ctx->condvars[idx];
>       pthread_mutex_t *mutex = ctx->mutexes[idx];
>       return pthread_cond_wait(cond, mutex);
>   }

You can get into potential deadlock situations here, if your not careful...




static ctx locks;


static obj a, b, c;


Thread A
----------

1: ctx_pthread_mutex_lock( &locks, &a );
2: ctx_pthread_mutex_lock( &locks, &b );
3: ctx_pthread_mutex_unlock( &locks, &b );
4: ctx_pthread_mutex_unlock( &locks, &a );



Thread B
----------

1: ctx_pthread_mutex_lock( &locks, &b );
2: ctx_pthread_mutex_lock( &locks, &c );
3: ctx_pthread_mutex_unlock( &locks, &c );
4: ctx_pthread_mutex_unlock( &locks, &b );




Lets say that the objects map to the following mutexs:

&a = mutex1
&b = mutex2
&c = mutex1



and the execution sequence happens to go like this:

A1
B1
A2 -- deadlocked!
....



?


;)

.