From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4B96592A.9060707@domain.hid> Date: Tue, 09 Mar 2010 15:20:26 +0100 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4B9527FD.3040806@domain.hid> <4B952D13.3080306@domain.hid> <4B952EFD.5020509@domain.hid> <4B95308A.1000309@domain.hid> In-Reply-To: <4B95308A.1000309@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: Jan Kiszka Cc: xenomai-core 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. 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. -- Gilles.