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Date: Thu, 28 Aug 2008 09:30:48 +0200
From: Jan Kiszka <jan.kiszka@domain.hid>
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Subject: Re: [Xenomai-core] [RFC][PATCH 2/3] Switch to handle-based fast
	mutex owners
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: xenomai-core <xenomai@xenomai.org>

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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);
>=20
> 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.

Jan


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