reading process acquired p->lock and released virtio lock in sleep(),
but before the process had set p->status to SLEEPING, because the
wakeup tested p->status without holding p's lock. Thus, wakeup can
complete without seeing any process SLEEPING and then p sets p->status
to SLEEPING.
Fix some other issues:
- Don't initialize proc lock in allocproc(), because freeproc() sets
np->state = UNUSED and allocproc() can choose np and calls initlock()
on the process's lock, releasing np's lock accidentally. Move
initializing proc's lock to init.
- Protect nextpid using ptable.lock (and move into its own function)
Some clean up:
- Don't acquire p->lock when it p is used in a private way (e.g., exit()/grow()).
- Move find_runnable() back into scheduler().
locking plan, which is a difficult to understand because ptable lock
protects many invariants. This implementation has a bug: once in a
while xv6 unlocks a proc lock that is locked by another core.
