Message from discussion
condvar wait / signal on arbitrary object
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NNTP-Posting-Date: Wed, 13 Jul 2005 00:44:41 -0500
From: "Chris Thomasson" <_no_damn_spam_cristom@_no_damn_comcast.net_spam>
Newsgroups: comp.programming.threads
References: <pan.2005.07.12.18.46.24.794166@ioplex.com> <db14gm$fam$1@news1brm.Central.Sun.COM> <pan.2005.07.13.01.58.01.178104@ioplex.com>
Subject: Re: condvar wait / signal on arbitrary object
Date: Tue, 12 Jul 2005 22:47:59 -0700
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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!
...
?
;)
|
