mysterious hang in pthread_create

Kostik Belousov kostikbel at
Sat Aug 30 18:45:17 UTC 2008

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:
> 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.
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