THE DESIGN OF THE LOCK MODULE
                            design.mps.lock
                              draft design
                             dsm 1995-11-21

PURPOSE

.purpose: Support the locking needs of the thread-safe design. In particular:
- Recursive locks
- Binary locks
- Recursive "global" lock that need not be allocated or initialized by the 
client
- Binary "global" lock that need not be allocated or initialized by the client

.context: The MPS has to be able to operate in a multi-threaded environment.  
The thread-safe design (design.mps.thread-safety) requires client-allocatable 
binary locks, a global binary lock and a global recursive lock.  An interface 
to client-allocatable recursive locks is also present to support any potential 
use, because of historic requirements, and because the implementation will 
presumably be necessary anyway for the global recursive lock. 


BACKGROUND

.need: In an environment where multiple threads are accessing shared data.  The 
threads which access data which is shared with other threads need to cooperate 
with those threads to maintain consistency.  Locks provide a simple mechanism 
for doing this.

.ownership: A lock is an object which may be "owned" by a single thread at a 
time.  By claiming ownership of a lock before executing some piece of code a 
thread can guarantee that no other thread owns the lock during execution of 
that code.  If some other thread holds a claim on a lock, the thread trying to 
claim the lock will suspend until the lock is released by the owning thread.

.data: A simple way of using this behaviour is to associate a lock with a 
shared data structure.  By claiming that lock around accesses to the data, a 
consistent view of the structure can be seen by the accessing thread.  More 
generally any set of operations which are required to be mutally exclusive may 
be performed so by using locks.


OVERVIEW

.adt: There is an ADT "Lock" which points to a locking structure "LockStruct".  
This structure is opaque to any client, although an interface is provided to 
supply the size of the structure for any client wishing to make a new lock.  
The lock is not allocated by the module as allocation itself may require 
locking.  LockStruct is implementation specific.

.simple-lock: There are facilities for claiming and releasing locks.  "Lock" is 
used for both binary and recursive locking.

.global-locks: "Global" locks are so called because they are used to protect 
data in a global location (such as a global variable). The lock module provides 
2 global locks; one recursive and one binary.  There are facilities for 
claiming and releasing both of these locks. These global locks have the 
advantage that they need not be allocated or atomically initialized by the 
client, so they may be used for locking the initialization of the allocator 
itself.  The binary global lock is intended to protect mutable data, possibly 
in conjunction with other local locking strategies.  The recursive global lock 
is intended to protect static read-only data during one-off initialization.  
See design.mps.thread-safety.

.deadlock: This module does not provide any deadlock protection.  Clients are 
responsible for avoiding deadlock by using traditional strategies such as 
ordering of locks.  (See design.mps.thread-safety.deadlock.)

.single-thread: In the single-threaded configuration, locks are not needed and 
the claim/release interfaces defined to be no-ops.


DETAILED DESIGN

.interface: The interface comprises the following functions:

- LockSize
  Return the size of a LockStruct for allocation purposes.

- LockInit / LockFinish
  After initialisation the lock is not owned by any thread.  This must also be 
the case before finalisation.
[ref convention?]

- LockClaim / LockRelease
  LockClaim: claims ownership of a lock that was previously not held by current 
thread.
  LockRelease: releases ownership of a lock that is currently owned.

- LockClaimRecursive / LockReleaseRecursive
  LockClaimRecursive: remembers the previous state of the lock with respect to 
the current thread and claims the lock (if not already held).
  LockReleaseRecursive: restores the previous state of the lock stored by 
corresponding LockClaimRecursive call.

- LockClaimGlobal / LockReleaseGlobal
  LockClaimGlobal: claims ownership of the binary global lock which was 
previously not held by current thread.
  LockReleaseGlobal: releases ownership of the binary global lock that is 
currently owned.

- LockClaimGlobalRecursive / LockReleaseGlobalRecursive
  LockClaimGlobalRecursive: remembers the previous state of the recursive 
global lock with respect to the current thread and claims the lock (if not 
already held).
  LockReleaseGlobalRecursive: restores the previous state of the recursive 
global lock stored by corresponding LockClaimGlobalRecursive call.

.impl.recursive: For recursive claims, the list of previous states can be 
simply implemented by keeping a count of the number of claims made by the 
current thread so far.  In multi-threaded implementation below this is handled 
by the operating system.  A count is still kept and used to check correctness.

.impl.global: The binary and recursive global locks may actually be implemented 
using the same mechanism as normal locks.

.impl.ansi: Single-Threaded Generic Implementation
- single-thread
- no need for locking
- locking structure contains count
- provides checking in debug version
- otherwise does nothing except keep count of claims

.impl.win32: Win32 Implementation
- supports Win32's threads
- uses Critical Sections [ref?]
- locking structure contains a Critical Section
- both recursive and non-recursive calls use same Windows function
- also performs checking

.impl.linux: LinuxThreads Implementation (possibly suitable for all PThreads 
implementations)
- supports LinuxThreads threads, which are an implementation of PThreads. (see 
<URL:http://pauillac.inria.fr/~xleroy/linuxthreads/>)
- locking structure contains a mutex, initialized to check for recursive locking
- locking structure contains a count of the number of active claims
- non-recursive locking calls pthread_mutex_lock and expects success
- recursive locking calls pthread_mutex_lock and expects either success or 
EDEADLK (indicating a recursive claim).
- also performs checking