umtx/libthr SMP fixes.

[Terry Lambert]

Scott Long wrote:
> Bryan Liesner wrote:
> It's very hard to imagine Jeff's patches causing a problem at the point
> that the PR mentions.  Have you confirmed the problem in a kernel that
> was build in a totally clean environment?

The changed code is not protecting a traversal of a proc
struct member with a proc lock in two places.  What's hard
to imagine?

Even if it's weren't necessary to protect that FOREACH loop
(it's necessary; but even if it weren't...), locking the
proc could easily serialize a number of operations through
that or other code which could save it from interference.

-- Terry