Re: [Xenomai-help] mutex locking

[Philippe Gerum <rpm@xenomai.org>]

On Wed, 2006-06-21 at 11:37 +0200, Tobias Bär wrote:
> Hi all,
> 
> my question is about the right way to lock and inquire a mutex. At time, my code looks like this:
> 
> 
> rt_mutex_inquire(&mutex,TM_NONBLOCK);
> 
> if(info.lockcnt == 0) 
> {
>       rt_mutex_lock(&write_mutex,TM_NONBLOCK);
> 
>       // do sth.
> 
>       rt_mutex_unlock(&mutex);
> }
> 
> 
> What happens if anyone locks the mutex between my "rt_mutex_inquire(...)" and my "if(info.lockcnt == 0)" ?
> 
> Is this the standard way to lock and inquire a mutex?

No, this snippet is terminally broken, I'm afraid. The mutex object is
precisely aimed at avoiding this kind of race conditions; the code above
would introduce them back.

Native skin mutexes are recursive by nature, so there is no need to
inquire for their current status. Additionally, passing TM_NONBLOCK
might cause mutex_lock() to return an error, if another thread holds it
already, so you need to check for the return value in any case. What you
likely want is, either:

if (rt_mutex_lock(...TM_INFINITE) < 0)
	bail_out();
/* work in critical section */
rt_mutex_unlock()

or,

if (rt_mutex_lock(...TM_NONBLOCK) < 0)
	resource_not_immediately_available();
/* work in critical section */
rt_mutex_unlock()


> 
> Thanks,
>   Tobias
> 
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-- 
Philippe.