libpthread prevent glxgears to run

This code :

if (m->m_flags & MUTEX_FLAGS_PRIVATE)
                PANIC("Recurse on a private mutex.");

at file "/usr/src/lib/libpthread/thread/thr_mutex.c" line 1002 prevent
glxgears to run with the following error :

Fatal error 'Recurse on a private mutex.' at line 1003 in file
/usr/src/lib/libpthread/thread/thr_mutex.c (errno = 35)

If the line is removed the glxgears is running well.

Which could be wrong; The glgears code or the libptrhead code ? And what the
use of that line ?

Regards,