Re: [Xenomai-core] [Xenomai-git] common: do not let u_mode exceptional cases leak out of current.[ch]

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

Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Jan Kiszka wrote:
>>> Gilles Chanteperdrix wrote:
>>>> Jan Kiszka wrote:
>>>>> xenomai-git-request@domain.hid wrote:
>>>>>> diff --git a/include/asm-generic/bits/current.h b/include/asm-generic/bits/current.h
>>>>>> index f0e569c..b9ac680 100644
>>>>>> --- a/include/asm-generic/bits/current.h
>>>>>> +++ b/include/asm-generic/bits/current.h
>>>>> ...
>>>>>
>>>>>> @@ -33,25 +33,16 @@ static inline xnhandle_t xeno_get_current(void)
>>>>>>  {
>>>>>>  	void *val = pthread_getspecific(xeno_current_key);
>>>>>>  
>>>>>> -	if (!val)
>>>>>> -		return XN_NO_HANDLE;
>>>>>> -
>>>>>> -	return (xnhandle_t)val;
>>>>>> +	return (xnhandle_t)val ?: xeno_slow_get_current();
>>>>>>  }
>>>>> So when used with normal Linux threads, this will always trigger the
>>>>> syscall of xeno_slow_get_current()?
>>>>>
>>>>>> diff --git a/src/rtdk/assert_context.c b/src/rtdk/assert_context.c
>>>>>> index bad19f3..f67bcd8 100644
>>>>>> --- a/src/rtdk/assert_context.c
>>>>>> +++ b/src/rtdk/assert_context.c
>>>>>> @@ -30,12 +30,9 @@ void assert_nrt(void)
>>>>>>  	xnthread_info_t info;
>>>>>>  	int err;
>>>>>>  
>>>>>> -	if (xeno_get_current() != XN_NO_HANDLE)
>>>>>> -		return;
>>>>>> +	if (unlikely(xeno_get_current() != XN_NO_HANDLE &&
>>>>>> +		     !(xeno_get_current_mode() & XNRELAX))) {
>>>>> Then please provide a different API for assert_nrt as this makes the
>>>>> service too heavy for common use.
>>>> It is a non real-time thread. Its performances are not critical. A
>>>> non-blocking syscall is not that heavy. Especially compared to the
>>>> execution time of malloc. Ok, will not argue on this any more, as
>>>> obviously our opinions differ in that area.
>>> Sorry, disagree. We are using it in mixed applications where both RT
>>> latency as well as non-RT throughput matters. The assert_nrt fast was
>>> designed to remain lightweight for both non-Xenomai threads as well as
>>> migrated threads.
>>>
>>>> The point is that NULL (XN_NO_HANDLE) means at the same time a freed
>>>> mode and a mode after the TSD cleanup was run. So, emitting a syscall in
>>>> that case is the simplest thing we can do. And no, I did not find it
>>>> expensive. But in fact, using an additional syscall, assert_nrt could be
>>>> implemented as a simple dummy syscall which returns 0 and asks the
>>>> caller to be put in primary mode (and it would be even lighter). So, I
>>>> guess if you did not implemented that way, it means that you already
>>>> wanted to avoid the syscall.
>>> Can't follow on this yet.
>> ...specifically as an unset current key is a bug for a Xenomai thread.
>> Every skin library is supposed to set it, so there should be no need for
>> falling back to a syscall for them, and there is no point in trying it
>> for non-Xenomai threads.
>>
>> So why this fallback? Does it simplify something else that I miss ATM?
> 
> After the TSD cleanup is run, the TSD are set to NULL. Actually, I
> assumed that when doing this change. I need to check.

That is the way I implemented it in the kernel-space skin. But reading
the spec again, I may have been overzealous:
http://www.opengroup.org/onlinepubs/000095399/functions/pthread_key_create.html

-- 
					    Gilles.