minix/kernel/proc.c

/* This file contains essentially all of the process and message handling.
 * Together with "mpx.s" it forms the lowest layer of the MINIX kernel.
 * There is one entry point from the outside:
 *
 *   sys_call: 	      a system call, i.e., the kernel is trapped with an INT
 *
 * As well as several entry points used from the interrupt and task level:
 *
 *   lock_notify:     notify a process of a system event
 *   lock_send:	      send a message to a process
 *   lock_enqueue:    put a process on one of the scheduling queues 
 *   lock_dequeue:    remove a process from the scheduling queues
 *
 * Changes:
 *   Aug 19, 2005     rewrote scheduling code  (Jorrit N. Herder)
 *   Jul 25, 2005     rewrote system call handling  (Jorrit N. Herder)
 *   May 26, 2005     rewrote message passing functions  (Jorrit N. Herder)
 *   May 24, 2005     new notification system call  (Jorrit N. Herder)
 *   Oct 28, 2004     nonblocking send and receive calls  (Jorrit N. Herder)
 *
 * The code here is critical to make everything work and is important for the
 * overall performance of the system. A large fraction of the code deals with
 * list manipulation. To make this both easy to understand and fast to execute 
 * pointer pointers are used throughout the code. Pointer pointers prevent
 * exceptions for the head or tail of a linked list. 
 *
 *  node_t *queue, *new_node;	// assume these as global variables
 *  node_t **xpp = &queue; 	// get pointer pointer to head of queue 
 *  while (*xpp != NULL) 	// find last pointer of the linked list
 *      xpp = &(*xpp)->next;	// get pointer to next pointer 
 *  *xpp = new_node;		// now replace the end (the NULL pointer) 
 *  new_node->next = NULL;	// and mark the new end of the list
 * 
 * For example, when adding a new node to the end of the list, one normally 
 * makes an exception for an empty list and looks up the end of the list for 
 * nonempty lists. As shown above, this is not required with pointer pointers.
 */

#include <minix/com.h>
#include <minix/callnr.h>
#include "kernel.h"
#include "proc.h"

/* Scheduling and message passing functions. The functions are available to 
 * other parts of the kernel through lock_...(). The lock temporarily disables 
 * interrupts to prevent race conditions. 
 */
FORWARD _PROTOTYPE( int mini_send, (struct proc *caller_ptr, int dst,
		message *m_ptr, unsigned flags));
FORWARD _PROTOTYPE( int mini_receive, (struct proc *caller_ptr, int src,
		message *m_ptr, unsigned flags));
FORWARD _PROTOTYPE( int mini_notify, (struct proc *caller_ptr, int dst));
FORWARD _PROTOTYPE( int deadlock, (int function,
		register struct proc *caller, int src_dst));
FORWARD _PROTOTYPE( void enqueue, (struct proc *rp));
FORWARD _PROTOTYPE( void dequeue, (struct proc *rp));
FORWARD _PROTOTYPE( void sched, (struct proc *rp, int *queue, int *front));
FORWARD _PROTOTYPE( void pick_proc, (void));

#define BuildMess(m_ptr, src, dst_ptr) \
	(m_ptr)->m_source = (src); 					\
	(m_ptr)->m_type = NOTIFY_FROM(src);				\
	(m_ptr)->NOTIFY_TIMESTAMP = get_uptime();			\
	switch (src) {							\
	case HARDWARE:							\
		(m_ptr)->NOTIFY_ARG = priv(dst_ptr)->s_int_pending;	\
		priv(dst_ptr)->s_int_pending = 0;			\
		break;							\
	case SYSTEM:							\
		(m_ptr)->NOTIFY_ARG = priv(dst_ptr)->s_sig_pending;	\
		priv(dst_ptr)->s_sig_pending = 0;			\
		break;							\
	}

#if (CHIP == INTEL)
#define CopyMess(s,sp,sm,dp,dm) \
	cp_mess(s, (sp)->p_memmap[D].mem_phys,	\
		 (vir_bytes)sm, (dp)->p_memmap[D].mem_phys, (vir_bytes)dm)
#endif /* (CHIP == INTEL) */

#if (CHIP == M68000)
/* M68000 does not have cp_mess() in assembly like INTEL. Declare prototype
 * for cp_mess() here and define the function below. Also define CopyMess. 
 */
#endif /* (CHIP == M68000) */

/*===========================================================================*
 *				sys_call				     * 
 *===========================================================================*/
PUBLIC int sys_call(call_nr, src_dst, m_ptr)
int call_nr;			/* system call number and flags */
int src_dst;			/* src to receive from or dst to send to */
message *m_ptr;			/* pointer to message in the caller's space */
{
/* System calls are done by trapping to the kernel with an INT instruction.
 * The trap is caught and sys_call() is called to send or receive a message
 * (or both). The caller is always given by 'proc_ptr'.
 */
  register struct proc *caller_ptr = proc_ptr;	/* get pointer to caller */
  int function = call_nr & SYSCALL_FUNC;	/* get system call function */
  unsigned flags = call_nr & SYSCALL_FLAGS;	/* get flags */
  int mask_entry;				/* bit to check in send mask */
  int group_size;				/* used for deadlock check */
  int result;					/* the system call's result */
  vir_clicks vlo, vhi;		/* virtual clicks containing message to send */

  /* Check if the process has privileges for the requested call. Calls to the 
   * kernel may only be SENDREC, because tasks always reply and may not block 
   * if the caller doesn't do receive(). 
   */
  if (! (priv(caller_ptr)->s_trap_mask & (1 << function)) || 
          (iskerneln(src_dst) && function != SENDREC
           && function != RECEIVE)) { 
#if DEBUG_ENABLE_IPC_WARNINGS
      kprintf("sys_call: trap %d not allowed, caller %d, src_dst %d\n", 
          function, proc_nr(caller_ptr), src_dst);
#endif
      return(ETRAPDENIED);		/* trap denied by mask or kernel */
  }
  
  /* Require a valid source and/ or destination process, unless echoing. */
  if (src_dst != ANY && function != ECHO) {
      if (! isokprocn(src_dst)) { 
#if DEBUG_ENABLE_IPC_WARNINGS
          kprintf("sys_call: invalid src_dst, src_dst %d, caller %d\n", 
              src_dst, proc_nr(caller_ptr));
#endif
          return(EBADSRCDST);		/* invalid process number */
      }
      if (isemptyn(src_dst)) {
#if DEBUG_ENABLE_IPC_WARNINGS
          kprintf("sys_call: dead src_dst; trap %d, from %d, to %d\n", 
              function, proc_nr(caller_ptr), src_dst);
#endif
	  return(EDEADSRCDST);
      }
  }

  /* If the call involves a message buffer, i.e., for SEND, RECEIVE, SENDREC, 
   * or ECHO, check the message pointer. This check allows a message to be 
   * anywhere in data or stack or gap. It will have to be made more elaborate 
   * for machines which don't have the gap mapped. 
   */
  if (function & CHECK_PTR) {	
      vlo = (vir_bytes) m_ptr >> CLICK_SHIFT;		
      vhi = ((vir_bytes) m_ptr + MESS_SIZE - 1) >> CLICK_SHIFT;
      if (vlo < caller_ptr->p_memmap[D].mem_vir || vlo > vhi ||
              vhi >= caller_ptr->p_memmap[S].mem_vir + 
              caller_ptr->p_memmap[S].mem_len) {
#if DEBUG_ENABLE_IPC_WARNINGS
          kprintf("sys_call: invalid message pointer, trap %d, caller %d\n",
          	function, proc_nr(caller_ptr));
#endif
          return(EFAULT); 		/* invalid message pointer */
      }
  }

  /* If the call is to send to a process, i.e., for SEND, SENDREC or NOTIFY,
   * verify that the caller is allowed to send to the given destination. 
   */
  if (function & CHECK_DST) {	
      if (! get_sys_bit(priv(caller_ptr)->s_ipc_to, nr_to_id(src_dst))) {
#if DEBUG_ENABLE_IPC_WARNINGS
          kprintf("sys_call: ipc mask denied trap %d from %d to %d\n",
          	function, proc_nr(caller_ptr), src_dst);
#endif
          return(ECALLDENIED);		/* call denied by ipc mask */
      }
  }

  /* Check for a possible deadlock for blocking SEND(REC) and RECEIVE. */
  if (function & CHECK_DEADLOCK) {
      if (group_size = deadlock(function, caller_ptr, src_dst)) {
#if DEBUG_ENABLE_IPC_WARNINGS
          kprintf("sys_call: trap %d from %d to %d deadlocked, group size %d\n",
              function, proc_nr(caller_ptr), src_dst, group_size);
#endif
          return(ELOCKED);
      }
  }

  /* Now check if the call is known and try to perform the request. The only
   * system calls that exist in MINIX are sending and receiving messages.
   *   - SENDREC: combines SEND and RECEIVE in a single system call
   *   - SEND:    sender blocks until its message has been delivered
   *   - RECEIVE: receiver blocks until an acceptable message has arrived
   *   - NOTIFY:  nonblocking call; deliver notification or mark pending
   *   - ECHO:    nonblocking call; directly echo back the message 
   */
  switch(function) {
  case SENDREC:
      /* A flag is set so that notifications cannot interrupt SENDREC. */
      priv(caller_ptr)->s_flags |= SENDREC_BUSY;
      /* fall through */
  case SEND:			
      result = mini_send(caller_ptr, src_dst, m_ptr, flags);
      if (function == SEND || result != OK) {	
          break;				/* done, or SEND failed */
      }						/* fall through for SENDREC */
  case RECEIVE:			
      if (function == RECEIVE)
          priv(caller_ptr)->s_flags &= ~SENDREC_BUSY;
      result = mini_receive(caller_ptr, src_dst, m_ptr, flags);
      break;
  case NOTIFY:
      result = mini_notify(caller_ptr, src_dst);
      break;
  case ECHO:
      CopyMess(caller_ptr->p_nr, caller_ptr, m_ptr, caller_ptr, m_ptr);
      result = OK;
      break;
  default:
      result = EBADCALL;			/* illegal system call */
  }

  /* Now, return the result of the system call to the caller. */
  return(result);
}

/*===========================================================================*
 *				deadlock				     * 
 *===========================================================================*/
PRIVATE int deadlock(function, cp, src_dst) 
int function;					/* trap number */
register struct proc *cp;			/* pointer to caller */
register int src_dst;				/* src or dst process */
{
/* Check for deadlock. This can happen if 'caller_ptr' and 'src_dst' have
 * a cyclic dependency of blocking send and receive calls. The only cyclic 
 * depency that is not fatal is if the caller and target directly SEND(REC)
 * and RECEIVE to each other. If a deadlock is found, the group size is 
 * returned. Otherwise zero is returned. 
 */
  register struct proc *xp;			/* process pointer */
  int group_size = 1;				/* start with only caller */
  int trap_flags;

  while (src_dst != ANY) { 			/* check while process nr */
      xp = proc_addr(src_dst);			/* follow chain of processes */
      group_size ++;				/* extra process in group */

      /* Check whether the last process in the chain has a depency. If it 
       * has not, the cycle cannot be closed and we are done.
       */
      if (xp->p_rts_flags & RECEIVING) {	/* xp has dependency */
          src_dst = xp->p_getfrom;		/* get xp's source */
      } else if (xp->p_rts_flags & SENDING) {	/* xp has dependency */
          src_dst = xp->p_sendto;		/* get xp's destination */
      } else {
	  return(0);				/* not a deadlock */
      }

      /* Now check if there is a cyclic dependency. For group sizes of two,  
       * a combination of SEND(REC) and RECEIVE is not fatal. Larger groups
       * or other combinations indicate a deadlock.  
       */
      if (src_dst == proc_nr(cp)) {		/* possible deadlock */
	  if (group_size == 2) {		/* caller and src_dst */
	      /* The function number is magically converted to flags. */
	      if ((xp->p_rts_flags ^ (function << 2)) & SENDING) { 
	          return(0);			/* not a deadlock */
	      }
	  }
          return(group_size);			/* deadlock found */
      }
  }
  return(0);					/* not a deadlock */
}

/*===========================================================================*
 *				mini_send				     * 
 *===========================================================================*/
PRIVATE int mini_send(caller_ptr, dst, m_ptr, flags)
register struct proc *caller_ptr;	/* who is trying to send a message? */
int dst;				/* to whom is message being sent? */
message *m_ptr;				/* pointer to message buffer */
unsigned flags;				/* system call flags */
{
/* Send a message from 'caller_ptr' to 'dst'. If 'dst' is blocked waiting
 * for this message, copy the message to it and unblock 'dst'. If 'dst' is
 * not waiting at all, or is waiting for another source, queue 'caller_ptr'.
 */
  register struct proc *dst_ptr = proc_addr(dst);
  register struct proc **xpp;

  /* Check if 'dst' is blocked waiting for this message. The destination's 
   * SENDING flag may be set when its SENDREC call blocked while sending.  
   */
  if ( (dst_ptr->p_rts_flags & (RECEIVING | SENDING)) == RECEIVING &&
       (dst_ptr->p_getfrom == ANY || dst_ptr->p_getfrom == caller_ptr->p_nr)) {
	/* Destination is indeed waiting for this message. */
	CopyMess(caller_ptr->p_nr, caller_ptr, m_ptr, dst_ptr,
		 dst_ptr->p_messbuf);
	if ((dst_ptr->p_rts_flags &= ~RECEIVING) == 0) enqueue(dst_ptr);
  } else if ( ! (flags & NON_BLOCKING)) {
	/* Destination is not waiting.  Block and dequeue caller. */
	caller_ptr->p_messbuf = m_ptr;
	if (caller_ptr->p_rts_flags == 0) dequeue(caller_ptr);
	caller_ptr->p_rts_flags |= SENDING;
	caller_ptr->p_sendto = dst;

	/* Process is now blocked.  Put in on the destination's queue. */
	xpp = &dst_ptr->p_caller_q;		/* find end of list */
	while (*xpp != NIL_PROC) xpp = &(*xpp)->p_q_link;	
	*xpp = caller_ptr;			/* add caller to end */
	caller_ptr->p_q_link = NIL_PROC;	/* mark new end of list */
  } else {
	return(ENOTREADY);
  }
  return(OK);
}

/*===========================================================================*
 *				mini_receive				     * 
 *===========================================================================*/
PRIVATE int mini_receive(caller_ptr, src, m_ptr, flags)
register struct proc *caller_ptr;	/* process trying to get message */
int src;				/* which message source is wanted */
message *m_ptr;				/* pointer to message buffer */
unsigned flags;				/* system call flags */
{
/* A process or task wants to get a message.  If a message is already queued,
 * acquire it and deblock the sender.  If no message from the desired source
 * is available block the caller, unless the flags don't allow blocking.  
 */
  register struct proc **xpp;
  register struct notification **ntf_q_pp;
  message m;
  int bit_nr;
  sys_map_t *map;
  bitchunk_t *chunk;
  int i, src_id, src_proc_nr;

  /* Check to see if a message from desired source is already available.
   * The caller's SENDING flag may be set if SENDREC couldn't send. If it is
   * set, the process should be blocked.
   */
  if (!(caller_ptr->p_rts_flags & SENDING)) {

    /* Check if there are pending notifications, except for SENDREC. */
    if (! (priv(caller_ptr)->s_flags & SENDREC_BUSY)) {

        map = &priv(caller_ptr)->s_notify_pending;
        for (chunk=&map->chunk[0]; chunk<&map->chunk[NR_SYS_CHUNKS]; chunk++) {

            /* Find a pending notification from the requested source. */ 
            if (! *chunk) continue; 			/* no bits in chunk */
            for (i=0; ! (*chunk & (1<<i)); ++i) {} 	/* look up the bit */
            src_id = (chunk - &map->chunk[0]) * BITCHUNK_BITS + i;
            if (src_id >= NR_SYS_PROCS) break;		/* out of range */
            src_proc_nr = id_to_nr(src_id);		/* get source proc */
#if DEBUG_ENABLE_IPC_WARNINGS
	    if(src_proc_nr == NONE) {
		kprintf("mini_receive: sending notify from NONE\n");
	    }
#endif
            if (src!=ANY && src!=src_proc_nr) continue;	/* source not ok */
            *chunk &= ~(1 << i);			/* no longer pending */

            /* Found a suitable source, deliver the notification message. */
	    BuildMess(&m, src_proc_nr, caller_ptr);	/* assemble message */
            CopyMess(src_proc_nr, proc_addr(HARDWARE), &m, caller_ptr, m_ptr);
            return(OK);					/* report success */
        }
    }

    /* Check caller queue. Use pointer pointers to keep code simple. */
    xpp = &caller_ptr->p_caller_q;
    while (*xpp != NIL_PROC) {
        if (src == ANY || src == proc_nr(*xpp)) {
	    /* Found acceptable message. Copy it and update status. */
	    CopyMess((*xpp)->p_nr, *xpp, (*xpp)->p_messbuf, caller_ptr, m_ptr);
            if (((*xpp)->p_rts_flags &= ~SENDING) == 0) enqueue(*xpp);
            *xpp = (*xpp)->p_q_link;		/* remove from queue */
            return(OK);				/* report success */
	}
	xpp = &(*xpp)->p_q_link;		/* proceed to next */
    }
  }

  /* No suitable message is available or the caller couldn't send in SENDREC. 
   * Block the process trying to receive, unless the flags tell otherwise.
   */
  if ( ! (flags & NON_BLOCKING)) {
      caller_ptr->p_getfrom = src;		
      caller_ptr->p_messbuf = m_ptr;
      if (caller_ptr->p_rts_flags == 0) dequeue(caller_ptr);
      caller_ptr->p_rts_flags |= RECEIVING;		
      return(OK);
  } else {
      return(ENOTREADY);
  }
}

/*===========================================================================*
 *				mini_notify				     * 
 *===========================================================================*/
PRIVATE int mini_notify(caller_ptr, dst)
register struct proc *caller_ptr;	/* sender of the notification */
int dst;				/* which process to notify */
{
  register struct proc *dst_ptr = proc_addr(dst);
  int src_id;				/* source id for late delivery */
  message m;				/* the notification message */

  /* Check to see if target is blocked waiting for this message. A process 
   * can be both sending and receiving during a SENDREC system call.
   */
  if ((dst_ptr->p_rts_flags & (RECEIVING|SENDING)) == RECEIVING &&
      ! (priv(dst_ptr)->s_flags & SENDREC_BUSY) &&
      (dst_ptr->p_getfrom == ANY || dst_ptr->p_getfrom == caller_ptr->p_nr)) {

      /* Destination is indeed waiting for a message. Assemble a notification 
       * message and deliver it. Copy from pseudo-source HARDWARE, since the
       * message is in the kernel's address space.
       */ 
      BuildMess(&m, proc_nr(caller_ptr), dst_ptr);
      CopyMess(proc_nr(caller_ptr), proc_addr(HARDWARE), &m, 
          dst_ptr, dst_ptr->p_messbuf);
      dst_ptr->p_rts_flags &= ~RECEIVING;	/* deblock destination */
      if (dst_ptr->p_rts_flags == 0) enqueue(dst_ptr);
      return(OK);
  } 

  /* Destination is not ready to receive the notification. Add it to the 
   * bit map with pending notifications. Note the indirectness: the system id 
   * instead of the process number is used in the pending bit map.
   */ 
  src_id = priv(caller_ptr)->s_id;
  set_sys_bit(priv(dst_ptr)->s_notify_pending, src_id); 
  return(OK);
}

/*===========================================================================*
 *				lock_notify				     *
 *===========================================================================*/
PUBLIC int lock_notify(src, dst)
int src;			/* sender of the notification */
int dst;			/* who is to be notified */
{
/* Safe gateway to mini_notify() for tasks and interrupt handlers. The sender
 * is explicitely given to prevent confusion where the call comes from. MINIX 
 * kernel is not reentrant, which means to interrupts are disabled after 
 * the first kernel entry (hardware interrupt, trap, or exception). Locking
 * is done by temporarily disabling interrupts. 
 */
  int result;

  /* Exception or interrupt occurred, thus already locked. */
  if (k_reenter >= 0) {
      result = mini_notify(proc_addr(src), dst); 
  }

  /* Call from task level, locking is required. */
  else {
      lock(0, "notify");
      result = mini_notify(proc_addr(src), dst); 
      unlock(0);
  }
  return(result);
}

/*===========================================================================*
 *				enqueue					     * 
 *===========================================================================*/
PRIVATE void enqueue(rp)
register struct proc *rp;	/* this process is now runnable */
{
/* Add 'rp' to one of the queues of runnable processes.  This function is 
 * responsible for inserting a process into one of the scheduling queues. 
 * The mechanism is implemented here.   The actual scheduling policy is
 * defined in sched() and pick_proc().
 */
  int q;	 				/* scheduling queue to use */
  int front;					/* add to front or back */

#if DEBUG_SCHED_CHECK
  check_runqueues("enqueue");
  if (rp->p_ready) kprintf("enqueue() already ready process\n");
#endif

  /* Determine where to insert to process. */
  sched(rp, &q, &front);

  /* Now add the process to the queue. */
  if (rdy_head[q] == NIL_PROC) {		/* add to empty queue */
      rdy_head[q] = rdy_tail[q] = rp; 		/* create a new queue */
      rp->p_nextready = NIL_PROC;		/* mark new end */
  } 
  else if (front) {				/* add to head of queue */
      rp->p_nextready = rdy_head[q];		/* chain head of queue */
      rdy_head[q] = rp;				/* set new queue head */
  } 
  else {					/* add to tail of queue */
      rdy_tail[q]->p_nextready = rp;		/* chain tail of queue */	
      rdy_tail[q] = rp;				/* set new queue tail */
      rp->p_nextready = NIL_PROC;		/* mark new end */
  }

  /* Now select the next process to run. */
  pick_proc();			

  kloadinfo.procs_enqueued++;

#if DEBUG_SCHED_CHECK
  rp->p_ready = 1;
  check_runqueues("enqueue");
#endif
}

/*===========================================================================*
 *				dequeue					     * 
 *===========================================================================*/
PRIVATE void dequeue(rp)
register struct proc *rp;	/* this process is no longer runnable */
{
/* A process must be removed from the scheduling queues, for example, because
 * it has blocked.  If the currently active process is removed, a new process
 * is picked to run by calling pick_proc().
 */
  register int q = rp->p_priority;		/* queue to use */
  register struct proc **xpp;			/* iterate over queue */
  register struct proc *prev_xp;

  /* Side-effect for kernel: check if the task's stack still is ok? */
  if (iskernelp(rp)) { 				
	if (*priv(rp)->s_stack_guard != STACK_GUARD)
		panic("stack overrun by task", proc_nr(rp));
  }

#if DEBUG_SCHED_CHECK
  check_runqueues("dequeue");
  if (! rp->p_ready) kprintf("dequeue() already unready process\n");
#endif

  /* Now make sure that the process is not in its ready queue. Remove the 
   * process if it is found. A process can be made unready even if it is not 
   * running by being sent a signal that kills it.
   */
  prev_xp = NIL_PROC;				
  for (xpp = &rdy_head[q]; *xpp != NIL_PROC; xpp = &(*xpp)->p_nextready) {

      if (*xpp == rp) {				/* found process to remove */
          *xpp = (*xpp)->p_nextready;		/* replace with next chain */
          if (rp == rdy_tail[q])		/* queue tail removed */
              rdy_tail[q] = prev_xp;		/* set new tail */
          if (rp == proc_ptr || rp == next_ptr)	/* active process removed */
              pick_proc();			/* pick new process to run */
          break;
      }
      prev_xp = *xpp;				/* save previous in chain */
  }

  kloadinfo.procs_enqueued--;
  
#if DEBUG_SCHED_CHECK
  rp->p_ready = 0;
  check_runqueues("dequeue");
  if(kloadinfo.procs_enqueued < 0)
	kprintf("%d processes enqueued\n", kloadinfo.procs_enqueued);
#endif
}

/*===========================================================================*
 *				sched					     * 
 *===========================================================================*/
PRIVATE void sched(rp, queue, front)
register struct proc *rp;			/* process to be scheduled */
int *queue;					/* return: queue to use */
int *front;					/* return: front or back */
{
/* This function determines the scheduling policy.  It is called whenever a
 * process must be added to one of the scheduling queues to decide where to
 * insert it.  As a side-effect the process' priority may be updated.  
 */
  static struct proc *prev_ptr = NIL_PROC;	/* previous without time */
  int time_left = (rp->p_ticks_left > 0);	/* quantum fully consumed */
  int penalty = 0;				/* change in priority */

  /* Check whether the process has time left. Otherwise give a new quantum 
   * and possibly raise the priority.  Processes using multiple quantums 
   * in a row get a lower priority to catch infinite loops in high priority
   * processes (system servers and drivers). 
   */
  if ( ! time_left) {				/* quantum consumed ? */
      rp->p_ticks_left = rp->p_quantum_size; 	/* give new quantum */
      if (prev_ptr == rp) penalty ++;		/* catch infinite loops */
      else penalty --; 				/* give slow way back */
      prev_ptr = rp;				/* store ptr for next */
  }

  /* Determine the new priority of this process. The bounds are determined
   * by IDLE's queue and the maximum priority of this process. Kernel task 
   * and the idle process are never changed in priority.
   */
  if (penalty != 0 && ! iskernelp(rp)) {
      rp->p_priority += penalty;		/* update with penalty */
      if (rp->p_priority < rp->p_max_priority)  /* check upper bound */ 
          rp->p_priority=rp->p_max_priority;
      else if (rp->p_priority > IDLE_Q-1)   	/* check lower bound */
      	  rp->p_priority = IDLE_Q-1;
  }

  /* If there is time left, the process is added to the front of its queue, 
   * so that it can immediately run. The queue to use simply is always the
   * process' current priority. 
   */
  *queue = rp->p_priority;
  *front = time_left;
}

/*===========================================================================*
 *				pick_proc				     * 
 *===========================================================================*/
PRIVATE void pick_proc()
{
/* Decide who to run now.  A new process is selected by setting 'next_ptr'.
 * When a billable process is selected, record it in 'bill_ptr', so that the 
 * clock task can tell who to bill for system time.
 */
  register struct proc *rp;			/* process to run */
  int q;					/* iterate over queues */

  /* Check each of the scheduling queues for ready processes. The number of
   * queues is defined in proc.h, and priorities are set in the task table.
   * The lowest queue contains IDLE, which is always ready.
   */
  for (q=0; q < NR_SCHED_QUEUES; q++) {	
      if ( (rp = rdy_head[q]) != NIL_PROC) {
          next_ptr = rp;			/* run process 'rp' next */
          if (priv(rp)->s_flags & BILLABLE)	 	
              bill_ptr = rp;			/* bill for system time */
          return;				 
      }
  }
}

/*===========================================================================*
 *				lock_send				     *
 *===========================================================================*/
PUBLIC int lock_send(dst, m_ptr)
int dst;			/* to whom is message being sent? */
message *m_ptr;			/* pointer to message buffer */
{
/* Safe gateway to mini_send() for tasks. */
  int result;
  lock(2, "send");
  result = mini_send(proc_ptr, dst, m_ptr, NON_BLOCKING);
  unlock(2);
  return(result);
}

/*===========================================================================*
 *				lock_enqueue				     *
 *===========================================================================*/
PUBLIC void lock_enqueue(rp)
struct proc *rp;		/* this process is now runnable */
{
/* Safe gateway to enqueue() for tasks. */
  lock(3, "enqueue");
  enqueue(rp);
  unlock(3);
}

/*===========================================================================*
 *				lock_dequeue				     *
 *===========================================================================*/
PUBLIC void lock_dequeue(rp)
struct proc *rp;		/* this process is no longer runnable */
{
/* Safe gateway to dequeue() for tasks. */
  if (k_reenter >= 0) {
	/* We're in an exception or interrupt, so don't lock (and.. 
	 * don't unlock).
	 */
	dequeue(rp);
  } else {
	lock(4, "dequeue");
	dequeue(rp);
	unlock(4);
  }
}