mips: Membar audit.

This change should be safe because it doesn't remove or weaken any
memory barriers, but does add, clarify, or strengthen barriers.

Goals:

- Make sure mutex_enter/exit and mutex_spin_enter/exit have
  acquire/release semantics.

- New macros make maintenance easier and purpose clearer:

  . SYNC_ACQ is for load-before-load/store barrier, and BDSYNC_ACQ
    for a branch delay slot -- currently defined as plain sync for MP
    and nothing, or nop, for UP; thus it is no weaker than SYNC and
    BDSYNC as currently defined, which is syncw on Octeon, plain sync
    on non-Octeon MP, and nothing/nop on UP.

    It is not clear to me whether load-then-syncw or ll/sc-then-syncw
    or even bare load provides load-acquire semantics on Octeon -- if
    no, this will fix bugs; if yes (like it is on SPARC PSO), we can
    relax SYNC_ACQ to be syncw or nothing later.

  . SYNC_REL is for load/store-before-store barrier -- currently
    defined as plain sync for MP and nothing for UP.

    It is not clear to me whether syncw-then-store is enough for
    store-release on Octeon -- if no, we can leave this as is; if
    yes, we can relax SYNC_REL to be syncw on Octeon.

  . SYNC_PLUNGER is there to flush clogged Cavium store buffers, and
    BDSYNC_PLUNGER for a branch delay slot -- syncw on Octeon,
    nothing or nop on non-Octeon.

    => This is not necessary (or, as far as I'm aware, sufficient)
       for acquire semantics -- it serves only to flush store buffers
       where stores might otherwise linger for hundreds of thousands
       of cycles, which would, e.g., cause spin locks to be held for
       unreasonably long durations.

  Newerish revisions of the MIPS ISA also have finer-grained sync
  variants that could be plopped in here.

Mechanism:

Insert these barriers in the right places, replacing only those where
the definition is currently equivalent, so this change is safe.

- Replace #ifdef _MIPS_ARCH_OCTEONP / syncw / #endif at the end of
  atomic_cas_* by SYNC_PLUNGER, which is `sync 4' (a.k.a. syncw) if
  __OCTEON__ and empty otherwise.

  => From what I can tell, __OCTEON__ is defined in at least as many
     contexts as _MIPS_ARCH_OCTEONP -- i.e., there are some Octeons
     with no _MIPS_ARCH_OCTEONP, but I don't know if any of them are
     relevant to us or ever saw the light of day outside Cavium; we
     seem to buid with `-march=octeonp' so this is unlikely to make a
     difference.  If it turns out that we do care, well, now there's
     a central place to make the distinction for sync instructions.

- Replace post-ll/sc SYNC by SYNC_ACQ in _atomic_cas_*, which are
  internal kernel versions used in sys/arch/mips/include/lock.h where
  it assumes they have load-acquire semantics.  Should move this to
  lock.h later, since we _don't_ define __HAVE_ATOMIC_AS_MEMBAR on
  MIPS and so the extra barrier might be costly.

- Insert SYNC_REL before ll/sc, and replace post-ll/sc SYNC by
  SYNC_ACQ, in _ucas_*, which is used without any barriers in futex
  code and doesn't mention barriers in the man page so I have to
  assume it is required to be a release/acquire barrier.

- Change BDSYNC to BDSYNC_ACQ in mutex_enter and mutex_spin_enter.
  This is necessary to provide load-acquire semantics -- unclear if
  it was provided already by syncw on Octeon, but it seems more
  likely that either (a) no sync or syncw is needed at all, or (b)
  syncw is not enough and sync is needed, since syncw is only a
  store-before-store ordering barrier.

- Insert SYNC_REL before ll/sc in mutex_exit and mutex_spin_exit.
  This is currently redundant with the SYNC already there, but
  SYNC_REL more clearly identifies the necessary semantics in case we
  want to define it differently on different systems, and having a
  sync in the middle of an ll/sc is a bit weird and possibly not a
  good idea, so I intend to (carefully) remove the redundant SYNC in
  a later change.

- Change BDSYNC to BDSYNC_PLUNGER at the end of mutex_exit.  This has
  no semantic change right now -- it's syncw on Octeon, sync on
  non-Octeon MP, nop on UP -- but we can relax it later to nop on
  non-Cavium MP.

- Leave LLSCSYNC in for now -- it is apparently there for a Cavium
  erratum, but I'm not sure what the erratum is, exactly, and I have
  no reference for it.  I suspect these can be safely removed, but we
  might have to double up some other syncw instructions -- Linux uses
  it only in store-release sequences, not at the head of every ll/sc.


(riastradh)