this is the current threading patchset, which accumulated up during the
past two weeks. It consists of a biggest set of changes from Roland, to
make threaded signals work. There were still tons of testcases and
boundary conditions (mostly in the signal/exit/ptrace area) that we did
not handle correctly.

Roland's thread-signal semantics/behavior/ptrace fixes:

 - fix signal delivery race with do_exit() => signals are re-queued to the
   'process' if do_exit() finds pending unhandled ones. This prevents
   signals getting lost upon thread-sys_exit().

 - a non-main thread has died on one processor and gone to TASK_ZOMBIE,
   but before it's gotten to release_task a sys_wait4 on the other
   processor reaps it.  It's only because it's ptraced that this gets
   through eligible_child.  Somewhere in there the main thread is also
   dying so it reparents the child thread to hit that case.  This means
   that there is a race where P might be totally invalid.

 - forget_original_parent is not doing the right thing when the group
   leader dies, i.e. reparenting threads to init when there is a zombie
   group leader.  Perhaps it doesn't matter for any practical purpose
   without ptrace, though it makes for ppid=1 for each thread in core
   dumps, which looks funny. Incidentally, SIGCHLD here really should be
   p->exit_signal.

 - one of the gdb tests makes a questionable assumption about what kill
   will do when it has some threads stopped by ptrace and others running.

exit races:

1. Processor A is in sys_wait4 case TASK_STOPPED considering task P.
   Processor B is about to resume P and then switch to it.

   While A is inside that case block, B starts running P and it clears
   P->exit_code, or takes a pending fatal signal and sets it to a new
   value. Depending on the interleaving, the possible failure modes are:
        a. A gets to its put_user after B has cleared P->exit_code
           => returns with WIFSTOPPED, WSTOPSIG==0
        b. A gets to its put_user after B has set P->exit_code anew
           => returns with e.g. WIFSTOPPED, WSTOPSIG==SIGKILL

   A can spend an arbitrarily long time in that case block, because
   there's getrusage and put_user that can take page faults, and
   write_lock'ing of the tasklist_lock that can block.  But even if it's
   short the race is there in principle.

2. This is new with NPTL, i.e. CLONE_THREAD.
   Two processors A and B are both in sys_wait4 case TASK_STOPPED
   considering task P.

   Both get through their tests and fetches of P->exit_code before either
   gets to P->exit_code = 0.  => two threads return the same pid from
   waitpid.

   In other interleavings where one processor gets to its put_user after
   the other has cleared P->exit_code, it's like case 1(a).


3. SMP races with stop/cont signals

   First, take:

        kill(pid, SIGSTOP);
        kill(pid, SIGCONT);

   or:

        kill(pid, SIGSTOP);
        kill(pid, SIGKILL);

   It's possible for this to leave the process stopped with a pending
   SIGCONT/SIGKILL.  That's a state that should never be possible.
   Moreover, kill(pid, SIGKILL) without any repetition should always be
   enough to kill a process.  (Likewise SIGCONT when you know it's
   sequenced after the last stop signal, must be sufficient to resume a
   process.)

4. take:

        kill(pid, SIGKILL);     // or any fatal signal
        kill(pid, SIGCONT);     // or SIGKILL

    it's possible for this to cause pid to be reaped with status 0
    instead of its true termination status.  The equivalent scenario
    happens when the process being killed is in an _exit call or a
    trap-induced fatal signal before the kills.

plus i've done stability fixes for bugs that popped up during
beta-testing, and minor tidying of Roland's changes:

 - a rare tasklist corruption during exec, causing some very spurious and
   colorful crashes.

 - a copy_process()-related dereference of already freed thread structure
   if hit with a SIGKILL in the wrong moment.

 - SMP spinlock deadlocks in the signal code

this patchset has been tested quite well in the 2.4 backport of the
threading changes - and i've done some stresstesting on 2.5.59 SMP as
well, and did an x86 UP testcompile + testboot as well.