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proc.c

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "riscv.h"
#include "spinlock.h"
#include "proc.h"
#include "defs.h"

struct cpu cpus[NCPU];

struct proc proc[NPROC];

struct proc *initproc;

int nextpid = 1;
struct spinlock pid_lock;

extern void forkret(void);
static void freeproc(struct proc *p);

extern char trampoline[];

TIP

helps ensure that wakeups of wait()ing parents are not lost. helps obey the memory model when using p->parent. must be acquired before any p->lock.

struct spinlock wait_lock;

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Allocate a page for each process's kernel stack. Map it high in memory, followed by an invalid guard page.

void
proc_mapstacks(pagetable_t kpgtbl)
{
  struct proc *p;
  
  for(p = proc; p < &proc[NPROC]; p++) {
    char *pa = kalloc();
    if(pa == 0)
      panic("kalloc");
    uint64 va = KSTACK((int) (p - proc));
    kvmmap(kpgtbl, va, (uint64)pa, PGSIZE, PTE_R | PTE_W);
  }
}

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initialize the proc table.

void
procinit(void)
{
  struct proc *p;
  
  initlock(&pid_lock, "nextpid");
  initlock(&wait_lock, "wait_lock");
  for(p = proc; p < &proc[NPROC]; p++) {
      initlock(&p->lock, "proc");
      p->state = UNUSED;
      p->kstack = KSTACK((int) (p - proc));
  }
}

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Must be called with interrupts disabled, to prevent race with process being moved to a different CPU.

int
cpuid()
{
  int id = r_tp();
  return id;
}

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Return this CPU's cpu struct. Interrupts must be disabled.

struct cpu*
mycpu(void)
{
  int id = cpuid();
  struct cpu *c = &cpus[id];
  return c;
}

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Return the current struct proc *, or zero if none.

struct proc*
myproc(void)
{
  push_off();
  struct cpu *c = mycpu();
  struct proc *p = c->proc;
  pop_off();
  return p;
}

int
allocpid()
{
  int pid;
  
  acquire(&pid_lock);
  pid = nextpid;
  nextpid = nextpid + 1;
  release(&pid_lock);

  return pid;
}

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Look in the process table for an UNUSED proc. If found, initialize state required to run in the kernel, and return with p->lock held. If there are no free procs, or a memory allocation fails, return 0.

static struct proc*
allocproc(void)
{
  struct proc *p;

  for(p = proc; p < &proc[NPROC]; p++) {
    acquire(&p->lock);
    if(p->state == UNUSED) {
      goto found;
    } else {
      release(&p->lock);
    }
  }
  return 0;

found:
  p->pid = allocpid();
  p->state = USED;

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Allocate a trapframe page.

  if((p->trapframe = (struct trapframe *)kalloc()) == 0){
    freeproc(p);
    release(&p->lock);
    return 0;
  }

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An empty user page table.

  p->pagetable = proc_pagetable(p);
  if(p->pagetable == 0){
    freeproc(p);
    release(&p->lock);
    return 0;
  }

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Set up new context to start executing at forkret, which returns to user space.

  memset(&p->context, 0, sizeof(p->context));
  p->context.ra = (uint64)forkret;
  p->context.sp = p->kstack + PGSIZE;

  return p;
}

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free a proc structure and the data hanging from it, including user pages. p->lock must be held.

static void
freeproc(struct proc *p)
{
  if(p->trapframe)
    kfree((void*)p->trapframe);
  p->trapframe = 0;
  if(p->pagetable)
    proc_freepagetable(p->pagetable, p->sz);
  p->pagetable = 0;
  p->sz = 0;
  p->pid = 0;
  p->parent = 0;
  p->name[0] = 0;
  p->chan = 0;
  p->killed = 0;
  p->xstate = 0;
  p->state = UNUSED;
}

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Create a user page table for a given process, with no user memory, but with trampoline and trapframe pages.

pagetable_t
proc_pagetable(struct proc *p)
{
  pagetable_t pagetable;

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An empty page table.

  pagetable = uvmcreate();
  if(pagetable == 0)
    return 0;

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map the trampoline code (for system call return) at the highest user virtual address. only the supervisor uses it, on the way to/from user space, so not PTE_U.

  if(mappages(pagetable, TRAMPOLINE, PGSIZE,
              (uint64)trampoline, PTE_R | PTE_X) < 0){
    uvmfree(pagetable, 0);
    return 0;
  }

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map the trapframe page just below the trampoline page, for trampoline.S.

  if(mappages(pagetable, TRAPFRAME, PGSIZE,
              (uint64)(p->trapframe), PTE_R | PTE_W) < 0){
    uvmunmap(pagetable, TRAMPOLINE, 1, 0);
    uvmfree(pagetable, 0);
    return 0;
  }

  return pagetable;
}

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Free a process's page table, and free the physical memory it refers to.

void
proc_freepagetable(pagetable_t pagetable, uint64 sz)
{
  uvmunmap(pagetable, TRAMPOLINE, 1, 0);
  uvmunmap(pagetable, TRAPFRAME, 1, 0);
  uvmfree(pagetable, sz);
}

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a user program that calls exec("/init") assembled from ../user/initcode.S od -t xC ../user/initcode

uchar initcode[] = {
  0x17, 0x05, 0x00, 0x00, 0x13, 0x05, 0x45, 0x02,
  0x97, 0x05, 0x00, 0x00, 0x93, 0x85, 0x35, 0x02,
  0x93, 0x08, 0x70, 0x00, 0x73, 0x00, 0x00, 0x00,
  0x93, 0x08, 0x20, 0x00, 0x73, 0x00, 0x00, 0x00,
  0xef, 0xf0, 0x9f, 0xff, 0x2f, 0x69, 0x6e, 0x69,
  0x74, 0x00, 0x00, 0x24, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00
};

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Set up first user process.

void
userinit(void)
{
  struct proc *p;

  p = allocproc();
  initproc = p;

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allocate one user page and copy initcode's instructions and data into it.

  uvmfirst(p->pagetable, initcode, sizeof(initcode));
  p->sz = PGSIZE;

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prepare for the very first "return" from kernel to user.

  p->trapframe->epc = 0;
  p->trapframe->sp = PGSIZE;

  safestrcpy(p->name, "initcode", sizeof(p->name));
  p->cwd = namei("/");

  p->state = RUNNABLE;

  release(&p->lock);
}

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Grow or shrink user memory by n bytes. Return 0 on success, -1 on failure.

int
growproc(int n)
{
  uint64 sz;
  struct proc *p = myproc();

  sz = p->sz;
  if(n > 0){
    if((sz = uvmalloc(p->pagetable, sz, sz + n, PTE_W)) == 0) {
      return -1;
    }
  } else if(n < 0){
    sz = uvmdealloc(p->pagetable, sz, sz + n);
  }
  p->sz = sz;
  return 0;
}

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Create a new process, copying the parent. Sets up child kernel stack to return as if from fork() system call.

int
fork(void)
{
  int i, pid;
  struct proc *np;
  struct proc *p = myproc();

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Allocate process.

  if((np = allocproc()) == 0){
    return -1;
  }

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Copy user memory from parent to child.

  if(uvmcopy(p->pagetable, np->pagetable, p->sz) < 0){
    freeproc(np);
    release(&np->lock);
    return -1;
  }
  np->sz = p->sz;

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copy saved user registers.

  *(np->trapframe) = *(p->trapframe);

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Cause fork to return 0 in the child.

  np->trapframe->a0 = 0;

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increment reference counts on open file descriptors.

  for(i = 0; i < NOFILE; i++)
    if(p->ofile[i])
      np->ofile[i] = filedup(p->ofile[i]);
  np->cwd = idup(p->cwd);

  safestrcpy(np->name, p->name, sizeof(p->name));

  pid = np->pid;

  release(&np->lock);

  acquire(&wait_lock);
  np->parent = p;
  release(&wait_lock);

  acquire(&np->lock);
  np->state = RUNNABLE;
  release(&np->lock);

  return pid;
}

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Pass p's abandoned children to init. Caller must hold wait_lock.

void
reparent(struct proc *p)
{
  struct proc *pp;

  for(pp = proc; pp < &proc[NPROC]; pp++){
    if(pp->parent == p){
      pp->parent = initproc;
      wakeup(initproc);
    }
  }
}

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Exit the current process. Does not return. An exited process remains in the zombie state until its parent calls wait().

void
exit(int status)
{
  struct proc *p = myproc();

  if(p == initproc)
    panic("init exiting");

TIP

Close all open files.

  for(int fd = 0; fd < NOFILE; fd++){
    if(p->ofile[fd]){
      struct file *f = p->ofile[fd];
      fileclose(f);
      p->ofile[fd] = 0;
    }
  }

  begin_op();
  iput(p->cwd);
  end_op();
  p->cwd = 0;

  acquire(&wait_lock);

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Give any children to init.

  reparent(p);

TIP

Parent might be sleeping in wait().

  wakeup(p->parent);
  
  acquire(&p->lock);

  p->xstate = status;
  p->state = ZOMBIE;

  release(&wait_lock);

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Jump into the scheduler, never to return.

  sched();
  panic("zombie exit");
}

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Wait for a child process to exit and return its pid. Return -1 if this process has no children.

int
wait(uint64 addr)
{
  struct proc *pp;
  int havekids, pid;
  struct proc *p = myproc();

  acquire(&wait_lock);

  for(;;){

TIP

Scan through table looking for exited children.

    havekids = 0;
    for(pp = proc; pp < &proc[NPROC]; pp++){
      if(pp->parent == p){

TIP

make sure the child isn't still in exit() or swtch().

        acquire(&pp->lock);

        havekids = 1;
        if(pp->state == ZOMBIE){

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Found one.

          pid = pp->pid;
          if(addr != 0 && copyout(p->pagetable, addr, (char *)&pp->xstate,
                                  sizeof(pp->xstate)) < 0) {
            release(&pp->lock);
            release(&wait_lock);
            return -1;
          }
          freeproc(pp);
          release(&pp->lock);
          release(&wait_lock);
          return pid;
        }
        release(&pp->lock);
      }
    }

TIP

No point waiting if we don't have any children.

    if(!havekids || killed(p)){
      release(&wait_lock);
      return -1;
    }

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Wait for a child to exit.

    sleep(p, &wait_lock);
  }
}

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Per-CPU process scheduler. Each CPU calls scheduler() after setting itself up. Scheduler never returns. It loops, doing: - choose a process to run. - swtch to start running that process. - eventually that process transfers control via swtch back to the scheduler.

void
scheduler(void)
{
  struct proc *p;
  struct cpu *c = mycpu();

  c->proc = 0;
  for(;;){

TIP

The most recent process to run may have had interrupts turned off; enable them to avoid a deadlock if all processes are waiting.

    intr_on();

    int found = 0;
    for(p = proc; p < &proc[NPROC]; p++) {
      acquire(&p->lock);
      if(p->state == RUNNABLE) {

TIP

Switch to chosen process. It is the process's job to release its lock and then reacquire it before jumping back to us.

        p->state = RUNNING;
        c->proc = p;
        swtch(&c->context, &p->context);

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Process is done running for now. It should have changed its p->state before coming back.

        c->proc = 0;
        found = 1;
      }
      release(&p->lock);
    }
    if(found == 0) {

TIP

nothing to run; stop running on this core until an interrupt.

      intr_on();
      asm volatile("wfi");
    }
  }
}

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Switch to scheduler. Must hold only p->lock and have changed proc->state. Saves and restores intena because intena is a property of this kernel thread, not this CPU. It should be proc->intena and proc->noff, but that would break in the few places where a lock is held but there's no process.

void
sched(void)
{
  int intena;
  struct proc *p = myproc();

  if(!holding(&p->lock))
    panic("sched p->lock");
  if(mycpu()->noff != 1)
    panic("sched locks");
  if(p->state == RUNNING)
    panic("sched running");
  if(intr_get())
    panic("sched interruptible");

  intena = mycpu()->intena;
  swtch(&p->context, &mycpu()->context);
  mycpu()->intena = intena;
}

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Give up the CPU for one scheduling round.

void
yield(void)
{
  struct proc *p = myproc();
  acquire(&p->lock);
  p->state = RUNNABLE;
  sched();
  release(&p->lock);
}

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A fork child's very first scheduling by scheduler() will swtch to forkret.

void
forkret(void)
{
  static int first = 1;

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Still holding p->lock from scheduler.

  release(&myproc()->lock);

  if (first) {

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File system initialization must be run in the context of a regular process (e.g., because it calls sleep), and thus cannot be run from main().

    fsinit(ROOTDEV);

    first = 0;

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ensure other cores see first=0.

    __sync_synchronize();
  }

  usertrapret();
}

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Atomically release lock and sleep on chan. Reacquires lock when awakened.

void
sleep(void *chan, struct spinlock *lk)
{
  struct proc *p = myproc();

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Must acquire p->lock in order to change p->state and then call sched. Once we hold p->lock, we can be guaranteed that we won't miss any wakeup (wakeup locks p->lock), so it's okay to release lk.

  acquire(&p->lock);
  release(lk);

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Go to sleep.

  p->chan = chan;
  p->state = SLEEPING;

  sched();

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Tidy up.

  p->chan = 0;

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Reacquire original lock.

  release(&p->lock);
  acquire(lk);
}

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Wake up all processes sleeping on chan. Must be called without any p->lock.

void
wakeup(void *chan)
{
  struct proc *p;

  for(p = proc; p < &proc[NPROC]; p++) {
    if(p != myproc()){
      acquire(&p->lock);
      if(p->state == SLEEPING && p->chan == chan) {
        p->state = RUNNABLE;
      }
      release(&p->lock);
    }
  }
}

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Kill the process with the given pid. The victim won't exit until it tries to return to user space (see usertrap() in trap.c).

int
kill(int pid)
{
  struct proc *p;

  for(p = proc; p < &proc[NPROC]; p++){
    acquire(&p->lock);
    if(p->pid == pid){
      p->killed = 1;
      if(p->state == SLEEPING){

TIP

Wake process from sleep().

        p->state = RUNNABLE;
      }
      release(&p->lock);
      return 0;
    }
    release(&p->lock);
  }
  return -1;
}

void
setkilled(struct proc *p)
{
  acquire(&p->lock);
  p->killed = 1;
  release(&p->lock);
}

int
killed(struct proc *p)
{
  int k;
  
  acquire(&p->lock);
  k = p->killed;
  release(&p->lock);
  return k;
}

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Copy to either a user address, or kernel address, depending on usr_dst. Returns 0 on success, -1 on error.

int
either_copyout(int user_dst, uint64 dst, void *src, uint64 len)
{
  struct proc *p = myproc();
  if(user_dst){
    return copyout(p->pagetable, dst, src, len);
  } else {
    memmove((char *)dst, src, len);
    return 0;
  }
}

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Copy from either a user address, or kernel address, depending on usr_src. Returns 0 on success, -1 on error.

int
either_copyin(void *dst, int user_src, uint64 src, uint64 len)
{
  struct proc *p = myproc();
  if(user_src){
    return copyin(p->pagetable, dst, src, len);
  } else {
    memmove(dst, (char*)src, len);
    return 0;
  }
}

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Print a process listing to console. For debugging. Runs when user types ^P on console. No lock to avoid wedging a stuck machine further.

void
procdump(void)
{
  static char *states[] = {
  [UNUSED]    "unused",
  [USED]      "used",
  [SLEEPING]  "sleep ",
  [RUNNABLE]  "runble",
  [RUNNING]   "run   ",
  [ZOMBIE]    "zombie"
  };
  struct proc *p;
  char *state;

  printf("\n");
  for(p = proc; p < &proc[NPROC]; p++){
    if(p->state == UNUSED)
      continue;
    if(p->state >= 0 && p->state < NELEM(states) && states[p->state])
      state = states[p->state];
    else
      state = "???";
    printf("%d %s %s", p->pid, state, p->name);
    printf("\n");
  }
}