Date: Sat, 30 Aug 2008 21:45:13 +0300 From: Kostik Belousov <kostikbel@gmail.com> To: Daniel Eischen <deischen@freebsd.org> Cc: davidxu@freebsd.org, Andriy Gapon <avg@icyb.net.ua>, freebsd-threads@freebsd.org Subject: Re: mysterious hang in pthread_create Message-ID: <20080830184512.GH2038@deviant.kiev.zoral.com.ua> In-Reply-To: <Pine.GSO.4.64.0808301200460.9898@sea.ntplx.net> 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>
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--vqZEy/DEMZDTzjXG Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sat, Aug 30, 2008 at 12:15:31PM -0400, Daniel Eischen wrote: > On Sat, 30 Aug 2008, Kostik Belousov wrote: >=20 > >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 implementati= on > >for rtld, then certainly yes. I looked over and patched only libthr > >since this is the only survived library for now. >=20 > 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. >=20 > libthr assumes that everything is single-threaded (or > serialized, I guess) before a thread is created. I > am looking at this thread: >=20 > http://docs.freebsd.org/cgi/getmsg.cgi?fetch=3D5235+0+current/freebsd-t= hreads >=20 > 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)? >=20 > 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". >=20 > > > >Anyway, I do not insist on the proposed solution, and definitely > >prefer the change that is well aligned with libthr architecture. >=20 > 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. --vqZEy/DEMZDTzjXG Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (FreeBSD) iEYEARECAAYFAki5lTgACgkQC3+MBN1Mb4j46wCgt5gz5qRSbHMdcx84LycxOFGT TP0AnRwctq3U++n1yqPYPY/qYqrb0rKN =RFyX -----END PGP SIGNATURE----- --vqZEy/DEMZDTzjXG--
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