From what I've seen as a TF for this course, it is very, very normal to write condition-variables code that looks like this:

struct cv {
    struct semaphore *sem;
    volatile int waiters;
}

void cv_wait(struct cv *cv, struct lock *lock) {
    KASSERT(lk_do_i_hold(lock));
    
    cv->waiters++;
    lk_release(lock);
    P(cv->sem);
    lk_acquire(lock);
}

void cv_broadcast(struct cv *cy struct lock *lock) {
    KASSERT(lk_do_i_hold(lock));
    
    for (; cv->waiters > 0; cv->waiters--)
        V(cv->sem);
}

This code is wrong (or, more specifically, badly synchronized). And it is such a common error that I'm choosing to dub it The Classic CV Error. It's subject to a race condition in e.g. the following case:

int before_and_after (int *the_thing, struct cv *thing_changed_cv, struct lock *thing_changed_lock) {
    lk_acquire(thing_changed_lock);
    
    int before = *the_thing;
    cv_wait(thing_changed_cv, thing_changed_lock);
    int after = *the_thing;
    
    KASSERT(before != after);
    
    lk_release(thing_changed_lock);
    return compare_things(before,after);
}

void mess_around (int *the_thing, struct cv *thing_changed_cv, struct lock *thing_changed_lock) {
    lk_acquire(thing_changed_lock);
    mutate(the_thing);
    
    cv_broadcast(thing_changed_cv, thing_changed_lock);
    
    lk_release(thing_changed_lock);
}

This code is properly synchronized (and in particular, protects the KASSERT(before != after)), assuming that your locks and CVs

Why Do We Schedule, Master Bruce?

A scheduler, as you know, is responsible for determining which threads run, for how long, and in what order. As much as possible, it should give the shared illusion that each process is running constantly to completion, using the entire processor. To this end, there are three major desiderata:

• That interactive threads (in particular, user-interactive threads) are responsive.
• That no process starves.
• That the system, on average, runs quickly.

These high-level desiderata factor into the low-level conditions that:

• Threads which block expecting a response are rescheduled promptly after waking.
• Time is allocated more-or-less fairly, subject to:
• Processes closer to completion are prioritized (recall that shortest-time-to-completion-first is provably optimal in total average time)
• ...but in any case, do not starve even the long-running processes too much (exponentially is a good benchmark -- since that, analytically, places a finite cap on the total runlength of a process).

Many of the designs I've seen from the class in A2 design documents fail in at least one of these respects, so I think it may be useful to go over common design failures and good designs.

indicates a potential issue of which you should be cautious. Check that your system doesn't have this as an issue!

Timeslicing and Timedicing

Not every thread blocks voluntarily; after running a thread for a certain