Unkillable KSE threaded proc

[John Baldwin]

On Thursday 16 September 2004 09:42 am, Andrew Gallatin wrote:
> Julian Elischer writes:
>  > Andrew, please try -current on ts own now..
>  > I have checked in some fixes that have helped others.
>
> OK, preemption off... Still a system lockup, but a little different.
>
> The interesting thing here is that continuing and breaking into the
> debugger repeatedly seems to show that thread 0xc1646af0 is looping in
> exit.  I've seen him in thread_single, thread_suspend_check, and in
> exit itself at kern_exit.c:163, etc.  A breakpoint in
> thread_suspend_one never triggers, so I guess he's holding the proc
> lock and just looping forever.  A breakpoint in _mtx_assert() shows
> him asserting the proc lock in thread_suspend_check at kern_thread.c:898.
> Over and over.

There is definitely some sort of infinite loop here.  Stripping out the 
comments in exit1() for that section of code reveals basically:

        PROC_LOCK(p);
        if (p->p_flag & P_HADTHREADS) {
retry:
                thread_suspend_check(0);
                if (thread_single(SINGLE_EXIT))
                        goto retry;
	}
        p->p_flag |= P_WEXIT;
        PROC_UNLOCK(p);

So it's easy to see how it can stuck in a loop I think.  If thread_single() 
never drops the lock then other threads that are waiting to die can't 
actually wait because they can never get the proc lock so that they can die.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org