main thread pthread_exit/sys_exit bug!

[Kaz Kylheku]

Basically, if you call pthread_exit from the main thread of a process, and keep
other threads running, the behavior is ugly.

I logged this initially as a bug against glibc, but then resolved it
with a kernel patch against linux 2.6.26:

Please see:

http://sources.redhat.com/bugzilla/show_bug.cgi?id=9804

I've known about this for some time, first having reproduced it on 2.6.17;
finally got around to fixing it.

When the main thread of a POSIX threads process calls pthread_exit, the process
should stick around until all the other threads do the same, or until one of
them calls _exit or exit, or until the process terminates abnormally.  During
this time, it would be nice if the process behaved normally: if it did not
appear defunct in the process list and if POSIX job control was possible on it.

An easy way to achieve this is to insert a wait into the top of sys_exit, so
that do_exit is not called unless all the other threads have terminated.  This
is another special case like do_group_exit. In the group exit, we zap the other
threads. In this case, we must not zap the other threads, but neither should we
fall through do_exit and become defunct!

The patch involves a controversial move: returning -ERESTARTSYS from sys_exit.
This is because the main thread may be stuck in sys_exit and have to respond to
a signal, and then go back to sys_exit. It appears to be working fine.
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