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Date: Wed, 27 Aug 2008 16:48:22 +0200
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-core] [RFC][PATCH 2/3] Switch to handle-based fast
	mutex owners
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To: Jan Kiszka <jan.kiszka@domain.hid>
Cc: xenomai-core <xenomai@xenomai.org>

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Jan Kiszka wrote:
>>
>>> -#define test_claimed(owner) ((long) (owner) & 1)
>>> -#define clear_claimed(owner) ((xnthread_t *) ((long) (owner) & ~1))
>>> -#define set_claimed(owner, bit) \
>>> -        ((xnthread_t *) ((long) clear_claimed(owner) | !!(bit)))
>>> +#define __CLAIMED_BIT		XN_HANDLE_SPARE3
>>> +
>>> +#define test_claimed(owner)	xnhandle_test_spare(owner, __CLAIMED_BIT)
>>> +#define clear_claimed(owner)	xnhandle_mask_spare(owner)
>>> +#define set_claimed(owner, bit) ({ \
>>> +	xnhandle_t __tmp = xnhandle_mask_spare(owner); \
>>> +	if (bit) \
>>> +		xnhandle_set_spare(__tmp, __CLAIMED_BIT); \
>>> +	__tmp; \
>>> +})
>> I liked doing this with no conditional.
> 
> You mean let the caller pass 'bit' as 0 or __CLAIMED_BIT (the latter
> would require some renaming then, I guess)?
> 
>>> +	xnarch_atomic_set(mutex->owner,
>>> +		 	  set_claimed(xnthread_handle(owner),
>>> +				      xnsynch_nsleepers(&mutex->synchbase)));
>> Ok. I think you have spotted a bug here. This should be mutex->sleepers
>> instead of xnsynch_nsleepers.
> 
> OK. Will you fix? I will rebase then.
> 
>>> +	/* Consistency check for owner handle - is the object a thread? */
>>> +	if (unlikely(xnthread_handle(owner) != clear_claimed(ownerh))) {
>>> +		err = -EINVAL;
>>> +		goto error;
>>>  	}
>> What is this ?
> 
> This ensures that we are not dereferences some arbitrary registry object
> due to a borken handle in the mutex variable: The the object registered
> on this handle is an xnthread, we will find the very same handle value
> at the well-known place in the object. Kind of magic for xnthread
> objects (instead of adding a magic field to them).
> 
>>
>>> Index: b/ksrc/skins/posix/thread.c
>>> ===================================================================
>>> --- a/ksrc/skins/posix/thread.c
>>> +++ b/ksrc/skins/posix/thread.c
>>> @@ -28,6 +28,7 @@
>>>   * 
>>>   *@{*/
>>>  
>>> +#include <nucleus/registry.h>
>>>  #include <posix/thread.h>
>>>  #include <posix/cancel.h>
>>>  #include <posix/timer.h>
>>> @@ -229,6 +230,13 @@ int pthread_create(pthread_t *tid,
>>>  	appendq(thread->container, &thread->link);
>>>  	xnlock_put_irqrestore(&nklock, s);
>>>  
>>> +#ifdef CONFIG_XENO_OPT_REGISTRY
>>> +	/* We need an anonymous registry entry to obtain a handle for fast
>>> +	   mutex locking. */
>>> +	xnregistry_enter("", &thread->threadbase,
>>> +			 &xnthread_handle(&thread->threadbase), NULL);
>>> +#endif /* CONFIG_XENO_OPT_REGISTRY */
>>> +
>> Since we need this for all threads (I want a thread from any skin to be
>> able to use the posix skin mutexes), why doing it in the skin ?
> 
> Some skins register their threads unconditionally, some based on names,
> some not at all. The "not at all" case has to be fixed, but we need to
> respect the other two as well.

Ok. But we need to add the register in all skins then.

-- 
                                                 Gilles.