Date: Sun, 31 Aug 2008 11:39:52 -0400 (EDT) From: Daniel Eischen <deischen@freebsd.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: davidxu@freebsd.org, Andriy Gapon <avg@icyb.net.ua>, freebsd-threads@freebsd.org Subject: Re: mysterious hang in pthread_create Message-ID: <Pine.GSO.4.64.0808311132280.15040@sea.ntplx.net> In-Reply-To: <20080830184512.GH2038@deviant.kiev.zoral.com.ua> References: <48B7101E.7060203@icyb.net.ua> <48B71BA6.5040504@icyb.net.ua> <20080829141043.GX2038@deviant.kiev.zoral.com.ua> <48B8052A.6070908@icyb.net.ua> <20080829143645.GY2038@deviant.kiev.zoral.com.ua> <Pine.GSO.4.64.0808291223240.5086@sea.ntplx.net> <20080829190506.GA2038@deviant.kiev.zoral.com.ua> <Pine.GSO.4.64.0808301128410.9898@sea.ntplx.net> <20080830155622.GF2038@deviant.kiev.zoral.com.ua> <Pine.GSO.4.64.0808301200460.9898@sea.ntplx.net> <20080830184512.GH2038@deviant.kiev.zoral.com.ua>
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On Sat, 30 Aug 2008, Kostik Belousov wrote: > On Sat, Aug 30, 2008 at 12:15:31PM -0400, Daniel Eischen wrote: >> On Sat, 30 Aug 2008, Kostik Belousov wrote: >> >>> On Sat, Aug 30, 2008 at 11:32:35AM -0400, Daniel Eischen wrote: >>>> On Fri, 29 Aug 2008, Kostik Belousov wrote: >>>>> >>>>> As demonstrated by Andriy' example, we need _thr_rtld_init() be called >>>>> before any rtld locks are given chance to be acquired. _thr_rtld_init() >>>>> shall be protected from repeated invocation, and _thr_setthreaded() >>>>> implements exactly this. >>>>> >>>>> If calling _thr_setthreaded(1) has not quite right intent, could you, >>>>> please, suggest satisfying solution ? >>>> >>>> I'm not sure I _quite_ understand the problem, but why >>>> wouldn't you have the same potential problem with some >>>> other library (without libthread)? I'll have to go back >>>> and read the beginning of the thread - I just kinda came >>>> in at the end. >>> >>> Sure, for appropriate value of any. If you mean whether the same problem >>> would arise for any threading library that supplies locking implementation >>> for rtld, then certainly yes. I looked over and patched only libthr >>> since this is the only survived library for now. >> >> What I mean is, is fixing libthr a solution that will work >> for cases? Or, is libthr doing something wrong? I can't >> really see that it is. >> >> libthr assumes that everything is single-threaded (or >> serialized, I guess) before a thread is created. I >> am looking at this thread: >> >> http://docs.freebsd.org/cgi/getmsg.cgi?fetch=5235+0+current/freebsd-threads >> >> Where is the corresponding unlock for the wlock_acquire()? >> I guess this is the problem. When would this normally >> be released (without libthr being linked in)? >> >> Also, the __isthreaded flag is used in libc to avoid taking >> locks unless necessary. So if you have a single threaded >> application that is also linked with libthr, you don't >> pay the penalty of locking overhead. Lots of 3rd-party >> libraries link with a threads library, so an application >> may not even know it is "threaded". >> >>> >>> Anyway, I do not insist on the proposed solution, and definitely >>> prefer the change that is well aligned with libthr architecture. >> >> I'm not arguing anything, I just don't know that the problem >> lies within lib<insert thread library here>. Of course, the >> rtld init stuff could be pulled out and done in thread >> initialization instead of thr_setthreaded(). That doesn't >> leave much in thr_setthreaded, and it also adds locking >> overhead into rtld for single-threaded programs that are >> linked with libthr... > > Ok, let me to tell the whole story. I am sure that in fact you know > it better then me. > > Assuming libthr is the only threading library, there are two locking > implementations for the rtld: 'default' and the one supplied by libthr. > On the first call to pthread_create(), libthr calls _rtld_thread_init() > to substitute the default by the implementation from libthr. > > In fact, default implementation is broken from my point of view. For > instance, thread_flag update is not atomic. Moreover, it does not > correctly handles sequential acquision of several locks, due > to thread_flag. > > The dl_iterate_phdr() function, called by gcc exception handling support > code, does exactly this. It acquires rtld_phdr_lock, then rtld_bind_lock. > [I shall admit it does this after my change]. In particular, this would > leave the bit for the bind lock set in the thread_flag. > > Andriy' example throw the exception and calls dl_iterate_phdr() before > first thread is created. On thread creation, _rtld_thread_init() is > called, that tries to move the locks according to thread_flag. This is > the cause for the reported wlock acquisition. > > I do not want to change anything in the default rtld locking. It is > disfunctional from the time libc_r is gone, and I think it would be > better to make it nop. My change makes the image that is linked with > libthr, to consistently use libthr locks. What happens if you remove the thread_flag() stuff (support for libc_r?) from rtld? It seems that libc_r should be providing its own rtld locking hooks - just like libthr does. -- DE
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