Re: [Xenomai-core] [RFC][PATCH 2/3] Switch to handle-based fast mutex owners

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>> @@ -329,6 +326,13 @@ int pse51_mutex_timedlock_break(struct _
>>>  				break;
>>>  			}
>>>  		}
>>> +		if (!xnsynch_nsleepers(&mutex->synchbase)) {
>>> +			xnarch_atomic_set
>>> +				(mutex->owner,
>>> +				 clear_claimed
>>> +					(xnarch_atomic_get(mutex->owner)));
>>> +			xnsynch_set_owner(&mutex->synchbase, NULL);
>>> +		}
>>>  		xnlock_put_irqrestore(&nklock, s);
>> I do not like this at all. I mean, unless I am mistaken, we loose more
>> than we gain, we are adding a couple of atomic, hence heavy, operations
>> in a common case for handling a corner case. I still prefer emitting a
>> system call in the corner case.
> 
> The hunk above is in the mutex' deadlock path - I wouldn't call this a
> common case. Moreover, we don't any expensive cmpxchg here.
> 
> I haven't counted ops, but my strong feeling is that this patch actually
> shortens the common code paths + it safely avoids one syscall in the
> lock stealing case.

I have not yet understood how this happens.

-- 
                                                 Gilles.