FAQ entry

Some applications don't themselves link against libpthread, but then load plugins which do link against libpthread. This means internally switching from single-threading to multi-threading, which is only supported since libc0.3 2.19-16~2. Previously, it would result in errors such as:

./pthread/../sysdeps/generic/pt-mutex-timedlock.c:70: __pthread_mutex_timedlock_internal: Assertion `__pthread_threads' failed.

This is now fixed as of libc0.3 2.19-16~2.

IRC, freenode, #hurd, 2010-01-24

<pinotree> youpi: hm, thought about the pthread/stubs issue w/ dlopen'ed
  libraries
<pinotree> currently looks like libstdc++ on hurd links to pthread-stubs,
  we're the only one with such configuration
<pinotree> i was looking at the gcc 4.4 patch hurd-pthread.diff, could it
  be it does not set THREADLIBS in the configure.ac switch case?
<youpi> that's expected
<youpi> on linux the libc provides hooks itself, on hurd-i386 it's
  pthread-stubs
<pinotree> why not explicitly link to pthread though?
<youpi> because there is no strict need to, for applications that don't
  need libpthread
<youpi> the dlopen case is a tricky case that pthread-stubs had not thought
  about
<pinotree> hm
<pinotree> what if the pthread stubs would be moved in our glibc?
<youpi> that's what we should do yes
<youpi> (ideally)
<youpi> but for this we need to build libpthread along glibc, to get it
  really working

Packaging libpthread.

<youpi> and that's the tricky part (Makefile & such) which hasn't been done
  yet
<pinotree> why both (stubs + actual libpthread)?
<youpi> because you need the stubs to be able to call the actual libpthread
<youpi> as soon libpthread gets dlopened for instance
<youpi> +as
<pinotree> i see
<youpi> (remember that nptl does this if you want to see how)
<youpi> (it's the libc files in nptl/)
<youpi> (and forward.c)
<guillem> also if libpthreads gets integrated with glibc don't we need to
  switch the hurd from cthreads then? Which has been the blocker all this
  time AFAIR?
<youpi> we don't _need_ to
<guillem> ok

IRC, OFTC, #debian-hurd, 2011-07-21

<youpi> there's one known issue with pthreads
<youpi> you can't dlopen() it

... if the main application is not already linked against it.

<youpi> which also means you can't dlopen() a module which depends on it if
  the main application hasn't used -lpthread already
<youpi> (so as to get libpthread initialized early, not at the dlopen()
  call)
<lucas> I get this while building simgrid:
<lucas> cd /home/lucas/simgrid-3.6.1/obj-i486-gnu/examples/gras/console &&
  /usr/bin/cmake -E create_symlink
  /home/lucas/simgrid-3.6.1/obj-i486-gnu/lib/libsimgrid.so
  /home/lucas/simgrid-3.6.1/obj-i486-gnu/examples/gras/console/simgrid.so
<lucas> cd /home/lucas/simgrid-3.6.1/obj-i486-gnu/examples/gras/console &&
  lua /home/lucas/simgrid-3.6.1/examples/gras/console/ping_generator.lua
<lucas> lua:
  /home/buildd/build/chroot-sid/home/buildd/byhand/hurd/./libpthread/sysdeps/generic/pt-mutex-timedlock.c:68:
  __pthread_mutex_timedlock_internal: Assertion `__pthread_threads' failed.
<lucas> Aborted (core dumped)
<youpi> that's it, yes
<youpi> (or at least it has the same symptoms)
<lucas> it would need fixing in lua, not in SG, then, right?
<youpi> yes
<lucas> ok, thanks

The fix thus being: link the main application with -lpthread.

IRC, freenode, #hurd, 2011-08-17

< youpi> i.e. openjade apparently dlopen()s modules which use pthreads, but
  openjade itself is not liked against libpthread
< youpi> which means unexpectedly  loading pthreads on the fly, which is
  not implemented
< youpi> (and hard to implement of course)
< youpi> gnu_srs: so simply tell openjade people to link it with -lpthread
< gnu_srs> Shuoldn't missing linking with pthread create an error when
  building openjade then?
< youpi> no
< youpi> because it's just a module which needs pthread
< youpi> and that module _is_ linked with -lpthread
< youpi> and dlopen() loads libpthreads too due to that
< youpi> but that's unexpected, for the libpthread initialization stuff
< youpi> (and too late to fix initlaization)
< gnu_srs> How come that other OSes build opensp w/o problems?
< youpi> because there are stubs in the libc
< gnu_srs> Sorry for the delay: What hinders stubs to be present also in
  the Hurd libc parts too, to cope with this problem?
< youpi> doing it
< youpi> which is hard because you need libpthread bits inside the libc
< youpi> making it simpler would need building libpthread at the same time
  as libc

packaging libpthread

IRC, freenode, #hurd, 2013-09-03

<gnu_srs> iceweasel: ./pthread/../sysdeps/generic/pt-mutex-timedlock.c:70:
  __pthread_mutex_timedlock_internal: Assertion `__pthread_threads' failed.
<pinotree> LD_PRELOAD libpthread
<gnu_srs> why
<pinotree> missing link to pthread?
<pinotree> and yes, it's known already, just nobody worked on solving it

IRC, freenode, #hurd, 2014-01-28

<gnu_srs> braunr: Is this fixed by your recent patches? test_dbi:
  ./pthread/../sysdeps/generic/pt-mutex-timedlock.c:70: 
<gnu_srs> __pthread_mutex_timedlock_internal: Assertion `__pthread_threads'
  failed.
<youpi> faq/libpthread_dlopen.mdwn:
  ./pthread/../sysdeps/generic/pt-mutex-timedlock.c:70:
  __pthread_mutex_time
<gnu_srs> youpi: tks. A workaround seems to be available:
  LD_PRELOAD=/lib/i386-gnu/libpthread.so.0.3
<gnu_srs> Is that possible on a buildd?
<youpi> it would be simpler to just make the package explicitly link
  libpthread
<gnu_srs> Package is libdbi-drivers, providing libdbd-sqlite3 needed by
  gnucash

IRC, freenode, #hurd, 2014-02-17

<braunr> hm ok, looks like iceweasel errors all have something to do with
  the libc dns resolver
<braunr> https://darnassus.sceen.net/~rbraun/iceweasel_crash
<braunr> apparently, it's simply because the memory chunk isn't page
  aligned ..
<braunr> looks like not preloading libpthread tirggers lots of tricky
  issues
<braunr> anyway, apparently, the malloc/free calls in libresolv don't use
  locks if libpthread isn't preloaded, which explains why the program state
  looked impossible to reach and why crashes look random
<congzhang> debian linux does not have the pthread load problem.
<braunr> congzhang: it had it
<braunr> maybe not debian but i've found one such report for opensuse

<braunr> ok the bug is simple
<braunr> for some reason, our glibc still uses a global _res state for dns
  resolution instead of per thread ones
<braunr> uh, apparently, it's libpthread's job to define a __res_state
  function for that :(

IRC, freenode, #hurd, 2014-02-18

<braunr> usually when i say it, it crashes soon after, so let's try it :
<braunr> i've been running iceweasel 27 fine for like 10 minutes with a
  patched libpthread
<braunr> still no crash ;p
<braunr> with luck this extremely lightweight patch will fix all
  multithreaded applications doing concurrent name resolution .... :)
<teythoon> nice :)
<braunr> let's try gnash ....
<braunr> uh, segfault on termination
<braunr> gnash works :)
<teythoon> sweet :)
<braunr> i'm very surprised we could live so long with that resolv bug

IRC, freenode, #hurd, 2014-02-19

<braunr> youpi: the eglibc bug is about libresolv
<braunr> it uses a global resolver state even in multithreaded applications
<youpi> libresolv is a horrible part of glibc :)
<braunr> which is obviously bad
<braunr> yes .. :)
<braunr> here is the patch :
  https://darnassus.sceen.net/~rbraun/0001-libpthread-per-thread-resolver-states.patch
<braunr> it's very short, it basically allocates a resolver state per
  thread in the pthread struct, and sets the TLS variable __resp when the
  thread starts
<braunr> should we make that hurd-specific ?
<braunr> or enclose that assignment with #ifdef ENABLE_TLS ?
<youpi> well, ENABLE_TLS is now always 1, iirc :)
<braunr> for the hurd, yes
<youpi> I'm surprised linux never had the issue
<youpi> no, not for the hurd
<braunr> ah
<youpi> I *had* to implement TLS for hurd because it was always 1 for
  everybody :)
<braunr> ok
<braunr> so all those ifdefs could be removed and libpthread can assume tls
  is enabled
<braunr> in which case my patch looks fine
<youpi> ah, thats a libpthread patch, not glibc patch
<braunr> yes
<braunr> nptl obviously did that from the start . :)
<braunr> linuxthreads had the problem a looong time ago
<youpi> ok
<braunr> i'm surprised we overlooked it for so long
<braunr> but anyway, that's a good fix
<youpi> indeed
<youpi> it seems all good to me
<braunr> well, __resp is a __thread variable
<braunr> i could add #ifdef ENABLE_TLS, but then what of the case where TLS
  isn't enabled, and do we actually care ?
<braunr> #error maybe ?
<braunr> or #warning ?
<youpi> I don't think we care about the non-TLS case any more
<braunr> ok
<braunr> topgit branch i suppose ?
<youpi> well, not, hurd libpthread repo :)
<braunr> oh right ... :)

libthreads vs. libpthread

The same symptom appears in an odd case, for instance:

buildd@hurd:~$ ldd /usr/bin/openjade
        libthreads.so.0.3 => /lib/libthreads.so.0.3 (0x0103d000)
        libosp.so.5 => /usr/lib/libosp.so.5 (0x01044000)
        libpthread.so.0.3 => /lib/libpthread.so.0.3 (0x01221000)
        libnsl.so.1 => /lib/i386-gnu/libnsl.so.1 (0x01232000)
[...]

openjade links against both libthreads and libpthread. The result is that libc early-initializes libthreads only, and thus libpthread is not early-initialized, and later on raises assertions. The solution is to just get rid of libthreads, to have only one threading library.