in main.c and don't make it disk specific; the icache is shared. This
doesn't matter since we have only one disk, but conceptually cleaner
and maybe helpful to students for mount lab.
procinit() and map them high up (below TRAMPOLNE) with an empty
mapping below each stack. Never free a kernel stack.
Another way would be to allocate and map them dynamically, but then we
need to reload page table when switching processes in scheduler()
and/or have a kernel pagetable per proc (if we want k->stack to be the
same virtual address in each process).
One gotcha: kernel addresses are not equal to physical addresses for
stack addresses. A stack address must be translated if we need its
physical address (e.g., virtio passes a stack address to the disk).
- during exit(), hold p's parent lock and p's lock across all changes
to p and its parent (e.g., reparenting and wakeup1). the lock
ordering between concurrent exits of children, parent, and great
parent might work out because processes form a tree.
- in wakeup1() test and set p->state atomically by asking caller to
have p locked.
a correctness proof would be desirable.
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.
