FreeBSD/Linux Kernel Cross Reference
sys/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();                  
   
   #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 */
     }
     
   #if DEBUG_SCHED_CHECK
     rp->p_ready = 0;
     check_runqueues("dequeue");
   #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. */
     lock(4, "dequeue");
     dequeue(rp);
     unlock(4);
   }
   

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