Date: Thu, 19 Nov 2009 15:00:19 -0500 (EST) From: Daniel Eischen <deischen@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: threads@freebsd.org Subject: Re: Using pthread_once() in libc Message-ID: <Pine.GSO.4.64.0911191458540.9230@sea.ntplx.net> In-Reply-To: <200911191254.50902.jhb@freebsd.org> References: <200911191030.14151.jhb@freebsd.org> <200911191202.30738.jhb@freebsd.org> <Pine.GSO.4.64.0911191204120.8401@sea.ntplx.net> <200911191254.50902.jhb@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thu, 19 Nov 2009, John Baldwin wrote: > On Thursday 19 November 2009 12:09:33 pm Daniel Eischen wrote: >> On Thu, 19 Nov 2009, John Baldwin wrote: >> >>> On Thursday 19 November 2009 11:48:54 am Daniel Eischen wrote: >>>> On Thu, 19 Nov 2009, John Baldwin wrote: >>>> >>>>> I would like to provide a pthread_once()-like facility in libc that library >>>>> bits can use to initialize data safely rather than trying to home-roll their >>>>> own variants (see the recent commit to stdtime in libc). Ideally what I >>>>> would like to do is have libc use the "real" pthread_once() when libthr is >>>>> linked in and fall back to a simple stub without libthr linked in. I know we >>>>> already do something like this for _spinlock() and friends. My question is >>>>> what is the most correct way to do this? Should libc grow a new _once() >>>>> symbol ala _spinlock() that is a weak symbol to a stub version and >>>>> pthread_once() in thr_once.c would override that, or should there be a >>>>> _pthread_once() in libc that is a stub in place of the current stub_zero? I >>>>> noticed a comment in thr_spinlock.c saying the spinlock stuff is kept for >>>>> backwards compat. Does this mean that for the future we would like to expose >>>>> pthread symbols directly in libc? Meaning would we rather have libc export a >>>>> pthread_once() and that ideally libc would be using pthread_mutex_lock/unlock >>>>> instead of _spinlock/unlock? >>>> >>>> pthread_once() is already a stub in libc that gets overloaded with the >>>> real thing when libthr is linked. See libc/gen/_pthread_stubs.c. >>>> Isn't that what you want or does it not serve your purpose? >>> >>> Hmm, the libc stub will never run the init routine. I would like to do >>> something like this: >> >> Well, I suppose you could do that. But what happens if libthr gets >> dlopen()'d and your once function needs to initialize a mutex or >> something that can only be properly done by a real threads library? >> Can we envision a scenario where that would be a problem? > > Hmmm, so I guess __is_threaded is how the dlopen() case is handled now for > mutex lock/unlock as that avoids resolving the symbol until pthread_create() > has been invoked? I guess then we could take an approach that works > something like this: > > /* libc-internal function */ > int > _once(pthread_once_t *once_control, void (*init_routine)(void)) > { > > if (__is_threaded) > return (_pthread_once(once_control, init_routine)); > > return (_stub_once(once_control, init_routine)); > } > > It is probably still a good idea to have the stub_once() patch I think so > that pthread_once() DTRT in a single-threaded program. I guess that works. -- DE
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?Pine.GSO.4.64.0911191458540.9230>