13.  Locking Programming Interfaces

  One of the major features of the Galaxy platform is the ability
  to share resources across multiple instances of the operat-
  ing system.  As with any shared resource, the need arises to
  synchronize access to that resource.  The services described
  in this chapter provide primitives upon which a coopera-
  tive scheme can be created to synchronize access to shared
  resources within a Galaxy.

  A galaxy lock   is a combination of a spinlock and a mu-
  tex.  While attempting to acquire an owned galaxy lock, the
  thread will spin for a short period.  If the lock does not be-
  come available during the spin, the thread will put itself into
  a wait state.  This is different from SMP spinlocks in which
  the system crashes if the spin times out, behavior that is not
  acceptable in a galaxy.

  Given the nature of galaxy locks, they will reside somewhere
  in shared memory.  That shared memory can be allocated
  either by the user or by the galaxy locking services.  If the
  user allocates the memory, the locking services track only
  the location of the locks.  If the locking services allocate the
  memory, it is managed on behalf of the user.

  Unlike other monitoring code which is only part of the MON
  version of execlets, the galaxy lock monitoring code is always
  loaded.

  There are several routines provided to manipulate galaxy
  locks.  The routines do not provide anything but the basics
  when it comes to locking.  They are a little richer than the
  spinlocks used to support SMP but far less than what the lock
  manager provides.