Re: [Xenomai-help] mutex locking

[Jan Kiszka <jan.kiszka@domain.hid>]

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Tobias Bär wrote:
>> -----Ursprüngliche Nachricht-----
>> Von: Jan Kiszka <jan.kiszka@domain.hid>
>> Gesendet: 21.06.06 11:46:34
>> An: Tobias Bär <Tobias.Baer@domain.hid>
>> CC: xenomai@xenomai.org
>> Betreff: Re: [Xenomai-help] mutex locking
> 
>> Try this, it's safe from such races:
>>
>> /* returns -EWOULDBLOCK if the mutex is locked */
>> if (rt_mutex_lock(&write_mutex,TM_NONBLOCK) == 0)
>> {
>>       // do sth.
>>
>>       rt_mutex_unlock(&mutex);
>> }
> 
> Thank's for the quick reply! But another question: How could it be that a mutex is locked twice?
> 

RTFM ;): "Xenomai mutexes are implicitely recursive [...]."

> if(rt_mutex_lock(&write_mutex,TM_NONBLOCK) == 0)
>   {
>     if(rt_mutex_lock(&write_mutex,TM_NONBLOCK) == 0)
>       printf("Mutex is locked twice!\n");
> 
>    rt_mutex_unlock(&mutex);
>   }
> 
> Is it locked by the ID of the thread?
> 

Yep, the owner is recorded, also to perform priority inheritance when
required. Recursive means that the current owner can acquire the mutex
multiple times (but (s)he also has to release it symmetrically then -
your example is lacking one unlock).

If you use the POSIX skin with its standard pthread_mutex'es, then you
can decide about the recursiveness explicitly.

Jan


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