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().