From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <48F499EF.9010205@domain.hid> Date: Tue, 14 Oct 2008 15:09:03 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <48F3C3E4.4020801@domain.hid> <48F45847.3030704@domain.hid> <48F461A6.6010600@domain.hid> <48F4634B.3030002@domain.hid> <48F465DD.6020505@domain.hid> <48F46954.6050704@domain.hid> <48F473A2.1050104@domain.hid> <48F48E2E.2050004@domain.hid> <48F49540.3060507@domain.hid> <48F49637.2030000@domain.hid> <48F497B2.4080404@domain.hid> <48F4987F.1060901@domain.hid> In-Reply-To: <48F4987F.1060901@domain.hid> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] __thread instead of pthread_get/setspecific List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: xenomai-core Gilles Chanteperdrix wrote: > Jan Kiszka wrote: >> Gilles Chanteperdrix wrote: >>> Jan Kiszka wrote: >>>> Gilles Chanteperdrix wrote: >>>>> Jan Kiszka wrote: >>>>>> Gilles Chanteperdrix wrote: >>>>>>> Jan Kiszka wrote: >>>>>>>> It will always remain orders of magnitude heavier than __thread >>>>>>>> variables which are a) inlined and b) should only need two memory >>>>>>>> accesses at worst. Moreover, it is clearly the future, while the >>>>>>>> importance of pthread_getspecific will decrease over the time. The >>>>>>>> __thread storage class is C99 standard (though its implementation >>>>>>>> remains a separate topic). >>>>>>> You are exagerating a bit: pthread_getspecific is pretty efficient >>>>>>> already (from the few things that I have timed on ARM, it is the only >>>>>>> one which takes under the microsecond). That you will gain something >>>>>>> with __thread is not guaranteed by the C99 standard either: in fact the >>>>>>> implementation could use exactly the same functions. >>>>>> As long as we do not loose anything (performance or portability), >>>>> You loose portability. But I agree that we do not care much. >>>> The fallback remains - must remain in order to obtain true optimization >>>> from the TLS-based version without locking out some corner-case usage. >>>> Find a proposal below (on top of handle-based xeno_get_current). >>>> >>>> We have to set initial-exec as TLS model, otherwise we end up with a >>>> dynamic lookup similar (maybe still faster, dunno) to the pthread >>>> service. This model requires start-time linking, will not work with >>>> dlopen (I strongly assume the linker will bail out). But I consider >>>> runtime loading of Xenomai libs as a uncommon corner case, and the user >>>> can still re-enable it via --without-__thread. >>> glibc has a separate test to know whether the tsl_model attribute is >>> supported: >>> >>> if test "$libc_cv_gcc___thread" = yes; then >>> dnl Check whether the compiler supports the tls_model attribute. >>> AC_CACHE_CHECK([for tls_model attribute], libc_cv_gcc_tls_model_attr, [dnl >>> cat > conftest.c <<\EOF >>> extern __thread int a __attribute__((tls_model ("initial-exec"))); >>> EOF >>> if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -S -Werror conftest.c >>>> &AS_MESSA >>> GE_LOG_FD]); then >>> libc_cv_gcc_tls_model_attr=yes >>> else >>> libc_cv_gcc_tls_model_attr=no >>> fi >>> rm -f conftest*]) >>> if test "$libc_cv_gcc_tls_model_attr" = yes; then >>> AC_DEFINE(HAVE_TLS_MODEL_ATTRIBUTE) >>> fi >>> fi >>> >> OK, but for us the question is if we want __thread without initial-exec >> at all. If not (I think so), I could add -Werror to the __thread test >> and that should combine both tests into a single one, sufficient for our >> use case. > > Yes, my point was that if an implementation supports __thread, it does > not necessarily mean that it supports the tls_model attribute. We should > fallback to pthread_key if either one is not supported. However, other > parts of Xenomai skins use pthread_specific, so, if we implement > something based on __thread, I think we should factor it and use it > everywhere. Ack. There are rt_printf (rtdk) and the task-self services of vxworks, vrtx and native skins. So if I get an OK for the proposal, I'll convert the rest, too. Jan -- Siemens AG, Corporate Technology, CT SE 2 Corporate Competence Center Embedded Linux