condvar wait / signal on arbitrary object - comp.programming.threads

Message from discussion condvar wait / signal on arbitrary object

Chris Thomasson

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More options Jul 13 2005, 1:47 am

Newsgroups: comp.programming.threads

From: "Chris Thomasson" <_no_damn_spam_cristom@_no_damn_comcast.net_spam>

Date: Tue, 12 Jul 2005 22:47:59 -0700

Local: Wed, Jul 13 2005 1:47 am

Subject: Re: condvar wait / signal on arbitrary object

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> 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!
...

;)

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