Re: Anomalies while handling p_nstopchild count

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Sun, 11 Oct 2015 09:45:04 +0800 (PHT)
   From: Paul Goyette <paul%vps1.whooppee.com@localhost>

   In kern/kern_exit.c at (original) line 231, there's a call to
   membar_producer(), presumably to ensure that p->p_waited is globally
   visible before updating p_.p_stat (which might cause someone to look
   at p->p_waited).

   Now that I'm planning to protect this whole area using proc_lock, I
   question whether or not the potentially expensive call to
   membar_producer() is still needed?

The membar_producer is certainly longer be needed.  It is not clear to
me why it was needed in the first place, nor is the original commit
message (kern_exit.c 1.158.2.6, `Checkpoint work in progress' on the
newlock2 branch) illuminating.  I see no matching membar_consumer,
even in the CVS logs.

My guess is that the membar_producer was an aborted attempt to avoid
locks or atomics for a certain part of the signal/wait protocol.