From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4B96600C.8060600@domain.hid> Date: Tue, 09 Mar 2010 15:49:48 +0100 From: Jan Kiszka MIME-Version: 1.0 References: <4B9527FD.3040806@domain.hid> <4B952D13.3080306@domain.hid> <4B952EFD.5020509@domain.hid> <4B95308A.1000309@domain.hid> <4B96592A.9060707@domain.hid> In-Reply-To: <4B96592A.9060707@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] [Xenomai-git] common: do not let u_mode exceptional cases leak out of current.[ch] List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: xenomai-core 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? > > So, to summarize, the problem here, is that current == XN_NO_HANDLE may > mean that the current thread is not a real-time thread, or that it is a > real-time thread, but that we are in the TSD cleanup, and the current > TSD was set to 0, as is done for TSD which have no destructor. > > We need get_current() to be correct, for the implementation of mutexes, > and in that case we want the additional syscall. Right, at least for !HAVE___THREAD. For HAVE___THREAD, I don't see a need for a syscall. Do you? > We need get_current() > to be correct for clock_gettime(), because as I understand, calling > linux' vdso clock_gettime may deadlock if we are not in secondary mode, > I do not know if you think that using a syscall for clock_gettime over > plain linux threads matters in that case. > > For the wrapping of malloc and free, I would say we do not really care > to absolutely get the real current. I do not think that a syscall for > each call to malloc and free is that prohibitive, their execution time > may already been long anyway, and the current implementation is correct. > It is only sub-optimal for your very peculiar case, so, I will let you > sweat on this issue. I will post a patch to add a "less-accurate" check for an assert_nrt_fast variant. As this only reduces accuracy for TSD destructor contexts, and that only under !HAVE___THREAD, I don't think it justifies the additional syscall for the majority of use cases of assert_nrt. But let's make the user choose the accuracy (s)he explicitly: assert_nrt for always correct results, assert_nrt_fast for syscall-less checks with known (and to-be-documented) limitations. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux