walk the list, we're looking for a vp to do something with. We do
this in the signal code and in the timer code. The signal code already
runs with proc::p_lock held, so it's a very natural lock to use. The
timer code, however, calls into the sa timer code with a spinlock held.
Since proc::p_lock is an adaptable mutex, we can sleep to get it. Sleeping
with a spinlock is BAD. So proc::p_lock is _not_ the right lock there,
and something like sadata::sa_mutex would be best.
Address this difficulty by noting that both uses actually just read
the list. Changing the list of VPs is rare - once one's added, it stays
until the process ends. So make the locking protocol that to write the
list you have to hold both proc::p_lock and sadata::sa_mutex (taken
in that order). Thus holding either one individually grants read access.
This removes a case where we could sleep with timer_lock, a spinlock at
IPL_SCHED (!!), while trying to get p_lock. If that ever happened, we'd
pretty much be dead. So don't do that!
This fixes a merge botch from how I handled our gaining p_lock - p_lock
should not have simply replaced p_smutex.
While here, tweak the sa_unblock_userret() code for the case
when the blessed vp is actually running (on another CPU). Make its
resched RESCHED_IMMED so we whack the CPU. Addresses a hang I've
observed in starting firefox on occasion when I see one thread running
in userland and another thread sitting in lwpublk, which means it's on
the list of threads for which we need an unblocked upcall. This list is
one on which things should NOT linger.
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