
                         +-----------------+
                         |     CS 140      |
                         |   SAMPLE TASK   |
                         | DESIGN DOCUMENT |
                         +-----------------+

---- GROUP ----

Ben Pfaff <blp@stanford.edu>

---- PRELIMINARIES ----

>> If you have any preliminary comments on your submission, notes for
>> the TAs, or extra credit, please give them here.

(This is a sample design document.)

>> Please cite any offline or online sources you consulted while
>> preparing your submission, other than the PintOS documentation,
>> course text, and lecture notes.

None.

                                 JOIN
                                 ====

---- DATA STRUCTURES ----

>> Copy here the declaration of each new or changed `struct' or `struct'
>> member, global or static variable, `typedef', or enumeration.
>> Identify the purpose of each in 25 words or less.

A "latch" is a new synchronization primitive.  Acquires block
until the first release.  Afterward, all ongoing and future
acquires pass immediately.

    /* Latch. */
    struct latch 
      {
        bool released;              /* Released yet? */
        struct lock monitor_lock;   /* Monitor lock. */
        struct condition rel_cond;  /* Signaled when released. */
      };

Added to struct thread:

    /* Members for implementing thread_join(). */
    struct latch ready_to_die;   /* Release when thread about to die. */
    struct semaphore can_die;    /* Up when thread allowed to die. */
    struct list children;        /* List of child threads. */
    list_elem children_elem;     /* Element of `children' list. */

---- ALGORITHMS ----

>> Briefly describe your implementation of thread_join() and how it
>> interacts with thread termination.

thread_join() finds the joined child on the thread's list of
children and waits for the child to exit by acquiring the child's
ready_to_die latch.  When thread_exit() is called, the thread
releases its ready_to_die latch, allowing the parent to continue.

---- SYNCHRONIZATION ----

>> Consider parent thread P with child thread C.  How do you ensure
>> proper synchronization and avoid race conditions when P calls wait(C)
>> before C exits?  After C exits?  How do you ensure that all resources
>> are freed in each case?  How about when P terminates without waiting,
>> before C exits?  After C exits?  Are there any special cases?

C waits in thread_exit() for P to die before it finishes its own
exit, using the can_die semaphore "down"ed by C and "up"ed by P as
it exits.  Regardless of whether whether C has terminated, there
is no race on wait(C), because C waits for P's permission before
it frees itself.

Regardless of whether P waits for C, P still "up"s C's can_die
semaphore when P dies, so C will always be freed.  (However,
freeing C's resources is delayed until P's death.)

The initial thread is a special case because it has no parent to
wait for it or to "up" its can_die semaphore.  Therefore, its
can_die semaphore is initialized to 1.

---- RATIONALE ----

>> Critique your design, pointing out advantages and disadvantages in
>> your design choices.

This design has the advantage of simplicity.  Encapsulating most
of the synchronization logic into a new "latch" structure
abstracts what little complexity there is into a separate layer,
making the design easier to reason about.  Also, all the new data
members are in `struct thread', with no need for any extra dynamic
allocation, etc., that would require extra management code.

On the other hand, this design is wasteful in that a child thread
cannot free itself before its parent has terminated.  A parent
thread that creates a large number of short-lived child threads
could unnecessarily exhaust kernel memory.  This is probably
acceptable for implementing kernel threads, but it may be a bad
idea for use with user processes because of the larger number of
resources that user processes tend to own.
