connect OSS to the build (clean and install only)

[minix.git] / 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_send:       send a message to a process
 *
 * 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 <minix/endpoint.h>
#include <stddef.h>
#include <signal.h>
#include <minix/portio.h>
#include <minix/u64.h>

#include "debug.h"
#include "kernel.h"
#include "proc.h"
#include "vm.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_e,
                message *m_ptr, int flags));
FORWARD _PROTOTYPE( int mini_receive, (struct proc *caller_ptr, int src,
                message *m_ptr, int flags));
FORWARD _PROTOTYPE( int mini_senda, (struct proc *caller_ptr,
        asynmsg_t *table, size_t size));
FORWARD _PROTOTYPE( int deadlock, (int function,
                register struct proc *caller, int src_dst));
FORWARD _PROTOTYPE( int try_async, (struct proc *caller_ptr));
FORWARD _PROTOTYPE( int try_one, (struct proc *src_ptr, struct proc *dst_ptr,
                int *postponed));
FORWARD _PROTOTYPE( void sched, (struct proc *rp, int *queue, int *front));
FORWARD _PROTOTYPE( void pick_proc, (void));

#define PICK_ANY        1
#define PICK_HIGHERONLY 2

#define BuildNotifyMessage(m_ptr, src, dst_ptr) \
        (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;                                                  \
        }

/*===========================================================================*
 *                              QueueMess                                    * 
 *===========================================================================*/
PRIVATE int QueueMess(endpoint_t ep, vir_bytes msg_lin, struct proc *dst)
{
        int k;
        phys_bytes addr;
        NOREC_ENTER(queuemess);
        /* Queue a message from the src process (in memory) to the dst
         * process (using dst process table entry). Do actual copy to
         * kernel here; it's an error if the copy fails into kernel.
         */
        vmassert(!(dst->p_misc_flags & MF_DELIVERMSG)); 
        vmassert(dst->p_delivermsg_lin);
        vmassert(isokendpt(ep, &k));

#if 0
        if(INMEMORY(dst)) {
                PHYS_COPY_CATCH(msg_lin, dst->p_delivermsg_lin,
                        sizeof(message), addr);
                if(!addr) {
                        PHYS_COPY_CATCH(vir2phys(&ep), dst->p_delivermsg_lin,
                                sizeof(ep), addr);
                        if(!addr) {
                                NOREC_RETURN(queuemess, OK);
                        }
                }
        }
#endif

        PHYS_COPY_CATCH(msg_lin, vir2phys(&dst->p_delivermsg), sizeof(message), addr);
        if(addr) {
                NOREC_RETURN(queuemess, EFAULT);
        }

        dst->p_delivermsg.m_source = ep;
        dst->p_misc_flags |= MF_DELIVERMSG;

        NOREC_RETURN(queuemess, OK);
}

/*===========================================================================*
 *                              schedcheck                                   * 
 *===========================================================================*/
PUBLIC void schedcheck(void)
{
        /* This function is called an instant before proc_ptr is
         * to be scheduled again.
         */
        NOREC_ENTER(schedch);
        vmassert(intr_disabled());
        if(next_ptr) {
                proc_ptr = next_ptr;
                next_ptr = NULL;
        }
        vmassert(proc_ptr);
        vmassert(!proc_ptr->p_rts_flags);
        while (proc_ptr->p_misc_flags &
                (MF_DELIVERMSG | MF_SC_DEFER | MF_SC_TRACE | MF_SC_ACTIVE)) {

                vmassert(!next_ptr);
                vmassert(!proc_ptr->p_rts_flags);
                if (proc_ptr->p_misc_flags & MF_DELIVERMSG) {
                        TRACE(VF_SCHEDULING, printf("delivering to %s / %d\n",
                                proc_ptr->p_name, proc_ptr->p_endpoint););
                        if(delivermsg(proc_ptr) == VMSUSPEND) {
                                vmassert(next_ptr);
                                TRACE(VF_SCHEDULING,
                                        printf("suspending %s / %d\n",
                                        proc_ptr->p_name,
                                        proc_ptr->p_endpoint););
                                vmassert(proc_ptr->p_rts_flags);
                                vmassert(next_ptr != proc_ptr);
                        }
                }
                else if (proc_ptr->p_misc_flags & MF_SC_DEFER) {
                        /* Perform the system call that we deferred earlier. */

#if DEBUG_SCHED_CHECK
                        if (proc_ptr->p_misc_flags & MF_SC_ACTIVE)
                                minix_panic("MF_SC_ACTIVE and MF_SC_DEFER set",
                                        NO_NUM);
#endif

                        arch_do_syscall(proc_ptr);

                        /* If the process is stopped for signal delivery, and
                         * not blocked sending a message after the system call,
                         * inform PM.
                         */
                        if ((proc_ptr->p_misc_flags & MF_SIG_DELAY) &&
                                        !RTS_ISSET(proc_ptr, SENDING))
                                sig_delay_done(proc_ptr);
                }
                else if (proc_ptr->p_misc_flags & MF_SC_TRACE) {
                        /* Trigger a system call leave event if this was a
                         * system call. We must do this after processing the
                         * other flags above, both for tracing correctness and
                         * to be able to use 'break'.
                         */
                        if (!(proc_ptr->p_misc_flags & MF_SC_ACTIVE))
                                break;

                        proc_ptr->p_misc_flags &=
                                ~(MF_SC_TRACE | MF_SC_ACTIVE);

                        /* Signal the "leave system call" event.
                         * Block the process.
                         */
                        cause_sig(proc_nr(proc_ptr), SIGTRAP);
                }
                else if (proc_ptr->p_misc_flags & MF_SC_ACTIVE) {
                        /* If MF_SC_ACTIVE was set, remove it now:
                         * we're leaving the system call.
                         */
                        proc_ptr->p_misc_flags &= ~MF_SC_ACTIVE;

                        break;
                }

                /* If proc_ptr is now descheduled,
                 * continue with another process.
                 */
                if (next_ptr) {
                        proc_ptr = next_ptr;
                        next_ptr = NULL;
                }
        }
        TRACE(VF_SCHEDULING, printf("starting %s / %d\n",
                proc_ptr->p_name, proc_ptr->p_endpoint););
#if DEBUG_TRACE
        proc_ptr->p_schedules++;
#endif
        NOREC_RETURN(schedch, );
}

/*===========================================================================*
 *                              sys_call                                     * 
 *===========================================================================*/
PUBLIC int sys_call(call_nr, src_dst_e, m_ptr, bit_map)
int call_nr;                    /* system call number and flags */
int src_dst_e;                  /* src to receive from or dst to send to */
message *m_ptr;                 /* pointer to message in the caller's space */
long bit_map;                   /* notification event set or flags */
{
/* 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 mask_entry;                               /* bit to check in send mask */
  int group_size;                               /* used for deadlock check */
  int result;                                   /* the system call's result */
  int src_dst_p;                                /* Process slot number */
  size_t msg_size;

  /* If this process is subject to system call tracing, handle that first. */
  if (caller_ptr->p_misc_flags & (MF_SC_TRACE | MF_SC_DEFER)) {
        /* Are we tracing this process, and is it the first sys_call entry? */
        if ((caller_ptr->p_misc_flags & (MF_SC_TRACE | MF_SC_DEFER)) ==
                                                        MF_SC_TRACE) {
                /* We must notify the tracer before processing the actual
                 * system call. If we don't, the tracer could not obtain the
                 * input message. Postpone the entire system call.
                 */
                caller_ptr->p_misc_flags &= ~MF_SC_TRACE;
                caller_ptr->p_misc_flags |= MF_SC_DEFER;

                /* Signal the "enter system call" event. Block the process. */
                cause_sig(proc_nr(caller_ptr), SIGTRAP);

                /* Preserve the return register's value. */
                return caller_ptr->p_reg.retreg;
        }

        /* If the MF_SC_DEFER flag is set, the syscall is now being resumed. */
        caller_ptr->p_misc_flags &= ~MF_SC_DEFER;

#if DEBUG_SCHED_CHECK
        if (caller_ptr->p_misc_flags & MF_SC_ACTIVE)
                minix_panic("MF_SC_ACTIVE already set", NO_NUM);
#endif

        /* Set a flag to allow reliable tracing of leaving the system call. */
        caller_ptr->p_misc_flags |= MF_SC_ACTIVE;
  }

#if DEBUG_SCHED_CHECK
  if(caller_ptr->p_misc_flags & MF_DELIVERMSG) {
        kprintf("sys_call: MF_DELIVERMSG on for %s / %d\n",
                caller_ptr->p_name, caller_ptr->p_endpoint);
        minix_panic("MF_DELIVERMSG on", NO_NUM);
  }
#endif

#if 0
  if(src_dst_e != 4 && src_dst_e != 5 &&
        caller_ptr->p_endpoint != 4 && caller_ptr->p_endpoint != 5) {
        if(call_nr == SEND)
                kprintf("(%d SEND to %d) ", caller_ptr->p_endpoint, src_dst_e);
        else if(call_nr == RECEIVE)
                kprintf("(%d RECEIVE from %d) ", caller_ptr->p_endpoint, src_dst_e);
        else if(call_nr == SENDREC)
                kprintf("(%d SENDREC to %d) ", caller_ptr->p_endpoint, src_dst_e);
        else
                kprintf("(%d %d to/from %d) ", caller_ptr->p_endpoint, call_nr, src_dst_e);
  }
#endif

#if DEBUG_SCHED_CHECK
  if (RTS_ISSET(caller_ptr, SLOT_FREE))
  {
        kprintf("called by the dead?!?\n");
        return EINVAL;
  }
#endif

  /* Check destination. SENDA is special because its argument is a table and
   * not a single destination. RECEIVE is the only call that accepts ANY (in
   * addition to a real endpoint). The other calls (SEND, SENDREC,
   * and NOTIFY) require an endpoint to corresponds to a process. In addition,
   * it is necessary to check whether a process is allowed to send to a given
   * destination.
   */
  if (call_nr == SENDA)
  {
        /* No destination argument */
  }
  else if (src_dst_e == ANY)
  {
        if (call_nr != RECEIVE)
        {
#if 0
                kprintf("sys_call: trap %d by %d with bad endpoint %d\n", 
                        call_nr, proc_nr(caller_ptr), src_dst_e);
#endif
                return EINVAL;
        }
        src_dst_p = src_dst_e;
  }
  else
  {
        /* Require a valid source and/or destination process. */
        if(!isokendpt(src_dst_e, &src_dst_p)) {
#if 0
                kprintf("sys_call: trap %d by %d with bad endpoint %d\n", 
                        call_nr, proc_nr(caller_ptr), src_dst_e);
#endif
                return EDEADSRCDST;
        }

        /* If the call is to send to a process, i.e., for SEND, SENDNB,
         * SENDREC or NOTIFY, verify that the caller is allowed to send to
         * the given destination. 
         */
        if (call_nr != RECEIVE)
        {
                if (!may_send_to(caller_ptr, src_dst_p)) {
#if DEBUG_ENABLE_IPC_WARNINGS
                        kprintf(
                        "sys_call: ipc mask denied trap %d from %d to %d\n",
                                call_nr, caller_ptr->p_endpoint, src_dst_e);
#endif
                        return(ECALLDENIED);    /* call denied by ipc mask */
                }
        }
  }

  /* Only allow non-negative call_nr values less than 32 */
  if (call_nr < 0 || call_nr >= 32)
  {
#if DEBUG_ENABLE_IPC_WARNINGS
      kprintf("sys_call: trap %d not allowed, caller %d, src_dst %d\n", 
          call_nr, proc_nr(caller_ptr), src_dst_p);
#endif
        return(ETRAPDENIED);            /* trap denied by mask or kernel */
  }

  /* 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 << call_nr))) {
#if DEBUG_ENABLE_IPC_WARNINGS
      kprintf("sys_call: trap %d not allowed, caller %d, src_dst %d\n", 
          call_nr, proc_nr(caller_ptr), src_dst_p);
#endif
        return(ETRAPDENIED);            /* trap denied by mask or kernel */
  }

  if ((iskerneln(src_dst_p) && call_nr != SENDREC && call_nr != RECEIVE)) {
#if DEBUG_ENABLE_IPC_WARNINGS
      kprintf("sys_call: trap %d not allowed, caller %d, src_dst %d\n", 
          call_nr, proc_nr(caller_ptr), src_dst_e);
#endif
        return(ETRAPDENIED);            /* trap denied by mask or kernel */
  }

  /* Get and check the size of the argument in bytes.
   * Normally this is just the size of a regular message, but in the
   * case of SENDA the argument is a table.
   */
  if(call_nr == SENDA) {
        msg_size = (size_t) src_dst_e;

        /* Limit size to something reasonable. An arbitrary choice is 16
         * times the number of process table entries.
         */
        if (msg_size > 16*(NR_TASKS + NR_PROCS))
                return EDOM;
        msg_size *= sizeof(asynmsg_t);  /* convert to bytes */
  } else {
        msg_size = sizeof(*m_ptr);
  }

  /* Check for a possible deadlock for blocking SEND(REC) and RECEIVE. */
  if (call_nr == SEND || call_nr == SENDREC || call_nr == RECEIVE) {
      if (group_size = deadlock(call_nr, caller_ptr, src_dst_p)) {
#if 0
          kprintf("sys_call: trap %d from %d to %d deadlocked, group size %d\n",
              call_nr, proc_nr(caller_ptr), src_dst_p, 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:  asynchronous call; deliver notification or mark pending
   *   - SENDA:   list of asynchronous send requests
   */
  switch(call_nr) {
  case SENDREC:
        /* A flag is set so that notifications cannot interrupt SENDREC. */
        caller_ptr->p_misc_flags |= MF_REPLY_PEND;
        /* fall through */
  case SEND:                    
        result = mini_send(caller_ptr, src_dst_e, m_ptr, 0);
        if (call_nr == SEND || result != OK)
                break;                          /* done, or SEND failed */
        /* fall through for SENDREC */
  case RECEIVE:                 
        if (call_nr == RECEIVE)
                caller_ptr->p_misc_flags &= ~MF_REPLY_PEND;
        result = mini_receive(caller_ptr, src_dst_e, m_ptr, 0);
        break;
  case NOTIFY:
        result = mini_notify(caller_ptr, src_dst_e);
        break;
  case SENDNB:
        result = mini_send(caller_ptr, src_dst_e, m_ptr, NON_BLOCKING);
        break;
  case SENDA:
        result = mini_senda(caller_ptr, (asynmsg_t *)m_ptr, (size_t)src_dst_e);
        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 */
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;
#if DEBUG_ENABLE_IPC_WARNINGS
  static struct proc *processes[NR_PROCS + NR_TASKS];
  processes[0] = cp;
#endif

  while (src_dst != ANY) {                      /* check while process nr */
      int src_dst_e;
      xp = proc_addr(src_dst);                  /* follow chain of processes */
#if DEBUG_ENABLE_IPC_WARNINGS
      processes[group_size] = xp;
#endif
      group_size ++;                            /* extra process in group */

      /* Check whether the last process in the chain has a dependency. If it 
       * has not, the cycle cannot be closed and we are done.
       */
      if (RTS_ISSET(xp, RECEIVING)) {   /* xp has dependency */
          if(xp->p_getfrom_e == ANY) src_dst = ANY;
          else okendpt(xp->p_getfrom_e, &src_dst);
      } else if (RTS_ISSET(xp, SENDING)) {      /* xp has dependency */
          okendpt(xp->p_sendto_e, &src_dst);
      } 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 */
              }
          }
#if DEBUG_ENABLE_IPC_WARNINGS
          {
                int i;
                kprintf("deadlock between these processes:\n");
                for(i = 0; i < group_size; i++) {
                        kprintf(" %10s ", processes[i]->p_name);
                        proc_stacktrace(processes[i]);
                }
          }
#endif
          return(group_size);                   /* deadlock found */
      }
  }
  return(0);                                    /* not a deadlock */
}

/*===========================================================================*
 *                              mini_send                                    * 
 *===========================================================================*/
PRIVATE int mini_send(caller_ptr, dst_e, m_ptr, flags)
register struct proc *caller_ptr;       /* who is trying to send a message? */
int dst_e;                              /* to whom is message being sent? */
message *m_ptr;                         /* pointer to message buffer */
int 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;
  register struct proc **xpp;
  int dst_p;
  phys_bytes linaddr;
  vir_bytes addr;
  int r;

  if(!(linaddr = umap_local(caller_ptr, D, (vir_bytes) m_ptr,
        sizeof(message)))) {
        return EFAULT;
  }
  dst_p = _ENDPOINT_P(dst_e);
  dst_ptr = proc_addr(dst_p);

  if (RTS_ISSET(dst_ptr, NO_ENDPOINT))
  {
        return EDSTDIED;
  }

  /* 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 (WILLRECEIVE(dst_ptr, caller_ptr->p_endpoint)) {
        /* Destination is indeed waiting for this message. */
        vmassert(!(dst_ptr->p_misc_flags & MF_DELIVERMSG));     
        if((r=QueueMess(caller_ptr->p_endpoint, linaddr, dst_ptr)) != OK)
                return r;
        RTS_UNSET(dst_ptr, RECEIVING);
  } else {
        if(flags & NON_BLOCKING) {
                return(ENOTREADY);
        }

        /* Destination is not waiting.  Block and dequeue caller. */
        PHYS_COPY_CATCH(linaddr, vir2phys(&caller_ptr->p_sendmsg),
                sizeof(message), addr);

        if(addr) { return EFAULT; }
        RTS_SET(caller_ptr, SENDING);
        caller_ptr->p_sendto_e = dst_e;

        /* 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 */
  }
  return(OK);
}

/*===========================================================================*
 *                              mini_receive                                 * 
 *===========================================================================*/
PRIVATE int mini_receive(caller_ptr, src_e, m_ptr, flags)
register struct proc *caller_ptr;       /* process trying to get message */
int src_e;                              /* which message source is wanted */
message *m_ptr;                         /* pointer to message buffer */
int 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.
 */
  register struct proc **xpp;
  register struct notification **ntf_q_pp;
  message m;
  int bit_nr;
  sys_map_t *map;
  bitchunk_t *chunk;
  int i, r, src_id, src_proc_nr, src_p;
  phys_bytes linaddr;

  vmassert(!(caller_ptr->p_misc_flags & MF_DELIVERMSG));

  if(!(linaddr = umap_local(caller_ptr, D, (vir_bytes) m_ptr,
        sizeof(message)))) {
        return EFAULT;
  }

  /* This is where we want our message. */
  caller_ptr->p_delivermsg_lin = linaddr;
  caller_ptr->p_delivermsg_vir = (vir_bytes) m_ptr;

  if(src_e == ANY) src_p = ANY;
  else
  {
        okendpt(src_e, &src_p);
        if (RTS_ISSET(proc_addr(src_p), NO_ENDPOINT))
        {
                return ESRCDIED;
        }
  }


  /* 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 (!RTS_ISSET(caller_ptr, SENDING)) {

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

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

            /* 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_e!=ANY && src_p != src_proc_nr) continue;/* source not ok */
            *chunk &= ~(1 << i);                        /* no longer pending */

            /* Found a suitable source, deliver the notification message. */
            BuildNotifyMessage(&m, src_proc_nr, caller_ptr);    /* assemble message */
            hisep = proc_addr(src_proc_nr)->p_endpoint;
            vmassert(!(caller_ptr->p_misc_flags & MF_DELIVERMSG));      
            vmassert(src_e == ANY || hisep == src_e);
            if((r=QueueMess(hisep, vir2phys(&m), caller_ptr)) != OK)  {
                minix_panic("mini_receive: local QueueMess failed", NO_NUM);
            }
            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_e == ANY || src_p == proc_nr(*xpp)) {
#if DEBUG_SCHED_CHECK
            if (RTS_ISSET(*xpp, SLOT_FREE) || RTS_ISSET(*xpp, NO_ENDPOINT))
            {
                kprintf("%d: receive from %d; found dead %d (%s)?\n",
                        caller_ptr->p_endpoint, src_e, (*xpp)->p_endpoint,
                        (*xpp)->p_name);
                return EINVAL;
            }
#endif

            /* Found acceptable message. Copy it and update status. */
            vmassert(!(caller_ptr->p_misc_flags & MF_DELIVERMSG));
            QueueMess((*xpp)->p_endpoint,
                vir2phys(&(*xpp)->p_sendmsg), caller_ptr);
            if ((*xpp)->p_misc_flags & MF_SIG_DELAY)
                sig_delay_done(*xpp);
            RTS_UNSET(*xpp, SENDING);
            *xpp = (*xpp)->p_q_link;            /* remove from queue */
            return(OK);                         /* report success */
        }
        xpp = &(*xpp)->p_q_link;                /* proceed to next */
    }

    if (caller_ptr->p_misc_flags & MF_ASYNMSG)
    {
        if (src_e != ANY)
                r= try_one(proc_addr(src_p), caller_ptr, NULL);
        else
                r= try_async(caller_ptr);

        if (r == OK)
                return OK;      /* Got a message */
    }
  }

  /* 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_e = src_e;          
      RTS_SET(caller_ptr, RECEIVING);
      return(OK);
  } else {
        return(ENOTREADY);
  }
}

/*===========================================================================*
 *                              mini_notify                                  * 
 *===========================================================================*/
PUBLIC int mini_notify(caller_ptr, dst_e)
register struct proc *caller_ptr;       /* sender of the notification */
endpoint_t dst_e;                       /* which process to notify */
{
  register struct proc *dst_ptr;
  int src_id;                           /* source id for late delivery */
  message m;                            /* the notification message */
  int r;
  int proc_nr;
  int dst_p;

  vmassert(intr_disabled());

  if (!isokendpt(dst_e, &dst_p)) {
        util_stacktrace();
        kprintf("mini_notify: bogus endpoint %d\n", dst_e);
        return EDEADSRCDST;
  }

  dst_ptr = proc_addr(dst_p);

  /* 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 (WILLRECEIVE(dst_ptr, caller_ptr->p_endpoint) &&
      ! (dst_ptr->p_misc_flags & MF_REPLY_PEND)) {
      /* 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.
       */ 
      BuildNotifyMessage(&m, proc_nr(caller_ptr), dst_ptr);
      vmassert(!(dst_ptr->p_misc_flags & MF_DELIVERMSG));
      if((r=QueueMess(caller_ptr->p_endpoint, vir2phys(&m), dst_ptr)) != OK) {
        minix_panic("mini_notify: local QueueMess failed", NO_NUM);
      }
      RTS_UNSET(dst_ptr, RECEIVING);
      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);
}

#define ASCOMPLAIN(caller, entry, field)        \
        kprintf("kernel:%s:%d: asyn failed for %s in %s "       \
        "(%d/%d, tab 0x%lx)\n",__FILE__,__LINE__,       \
field, caller->p_name, entry, priv(caller)->s_asynsize, priv(caller)->s_asyntab)

#define A_RETRIEVE(entry, field)        \
  if(data_copy(caller_ptr->p_endpoint,  \
         table_v + (entry)*sizeof(asynmsg_t) + offsetof(struct asynmsg,field),\
                SYSTEM, (vir_bytes) &tabent.field,      \
                        sizeof(tabent.field)) != OK) {\
                ASCOMPLAIN(caller_ptr, entry, #field);  \
                return EFAULT; \
        }

#define A_INSERT(entry, field)  \
  if(data_copy(SYSTEM, (vir_bytes) &tabent.field, \
        caller_ptr->p_endpoint, \
        table_v + (entry)*sizeof(asynmsg_t) + offsetof(struct asynmsg,field),\
                sizeof(tabent.field)) != OK) {\
                ASCOMPLAIN(caller_ptr, entry, #field);  \
                return EFAULT; \
        }

/*===========================================================================*
 *                              mini_senda                                   *
 *===========================================================================*/
PRIVATE int mini_senda(caller_ptr, table, size)
struct proc *caller_ptr;
asynmsg_t *table;
size_t size;
{
        int i, dst_p, done, do_notify, r;
        unsigned flags;
        struct proc *dst_ptr;
        struct priv *privp;
        message *m_ptr;
        asynmsg_t tabent;
        vir_bytes table_v = (vir_bytes) table;
        vir_bytes linaddr;

        privp= priv(caller_ptr);
        if (!(privp->s_flags & SYS_PROC))
        {
                kprintf(
                "mini_senda: warning caller has no privilege structure\n");
                return EPERM;
        }

        /* Clear table */
        privp->s_asyntab= -1;   
        privp->s_asynsize= 0;

        if (size == 0)
        {
                /* Nothing to do, just return */
                return OK;
        }

        if(!(linaddr = umap_local(caller_ptr, D, (vir_bytes) table,
                size * sizeof(*table)))) {
                printf("mini_senda: umap_local failed; 0x%lx len 0x%lx\n",
                        table, size * sizeof(*table));
                return EFAULT;
        }

        /* Limit size to something reasonable. An arbitrary choice is 16
         * times the number of process table entries.
         *
         * (this check has been duplicated in sys_call but is left here
         * as a sanity check)
         */
        if (size > 16*(NR_TASKS + NR_PROCS))
        {
                return EDOM;
        }
        
        /* Scan the table */
        do_notify= FALSE;       
        done= TRUE;
        for (i= 0; i<size; i++)
        {

                /* Read status word */
                A_RETRIEVE(i, flags);
                flags= tabent.flags;

                /* Skip empty entries */
                if (flags == 0)
                        continue;

                /* Check for reserved bits in the flags field */
                if (flags & ~(AMF_VALID|AMF_DONE|AMF_NOTIFY|AMF_NOREPLY) ||
                        !(flags & AMF_VALID))
                {
                        return EINVAL;
                }

                /* Skip entry if AMF_DONE is already set */
                if (flags & AMF_DONE)
                        continue;

                /* Get destination */
                A_RETRIEVE(i, dst);

                if (!isokendpt(tabent.dst, &dst_p))
                {
                        /* Bad destination, report the error */
                        tabent.result= EDEADSRCDST;
                        A_INSERT(i, result);
                        tabent.flags= flags | AMF_DONE;
                        A_INSERT(i, flags);

                        if (flags & AMF_NOTIFY)
                                do_notify= 1;
                        continue;
                }

                if (!may_send_to(caller_ptr, dst_p))
                {
                        /* Send denied by IPC mask */
                        tabent.result= ECALLDENIED;
                        A_INSERT(i, result);
                        tabent.flags= flags | AMF_DONE;
                        A_INSERT(i, flags);

                        if (flags & AMF_NOTIFY)
                                do_notify= 1;
                        continue;
                }

#if 0
                kprintf("mini_senda: entry[%d]: flags 0x%x dst %d/%d\n",
                        i, tabent.flags, tabent.dst, dst_p);
#endif

                dst_ptr = proc_addr(dst_p);

                /* NO_ENDPOINT should be removed */
                if (dst_ptr->p_rts_flags & NO_ENDPOINT)
                {
                        tabent.result= EDSTDIED;
                        A_INSERT(i, result);
                        tabent.flags= flags | AMF_DONE;
                        A_INSERT(i, flags);

                        if (flags & AMF_NOTIFY)
                                do_notify= TRUE;
                        continue;
                }

                /* Check if 'dst' is blocked waiting for this message.
                 * If AMF_NOREPLY is set, do not satisfy the receiving part of
                 * a SENDREC.
                 */
                if (WILLRECEIVE(dst_ptr, caller_ptr->p_endpoint) &&
                        (!(flags & AMF_NOREPLY) ||
                        !(dst_ptr->p_misc_flags & MF_REPLY_PEND)))
                {
                        /* Destination is indeed waiting for this message. */
                        m_ptr= &table[i].msg;   /* Note: pointer in the
                                                 * caller's address space.
                                                 */
                        /* Copy message from sender. */
                        tabent.result= QueueMess(caller_ptr->p_endpoint,
                                linaddr + (vir_bytes) &table[i].msg -
                                        (vir_bytes) table, dst_ptr);
                        if(tabent.result == OK)
                                RTS_UNSET(dst_ptr, RECEIVING);

                        A_INSERT(i, result);
                        tabent.flags= flags | AMF_DONE;
                        A_INSERT(i, flags);

                        if (flags & AMF_NOTIFY)
                                do_notify= 1;
                        continue;
                }
                else 
                {
                        /* Should inform receiver that something is pending */
                        dst_ptr->p_misc_flags |= MF_ASYNMSG;
                        done= FALSE;
                        continue;
                } 
        }
        if (do_notify)
                kprintf("mini_senda: should notify caller\n");
        if (!done)
        {
                privp->s_asyntab= (vir_bytes)table;
                privp->s_asynsize= size;
        }
        return OK;
}


/*===========================================================================*
 *                              try_async                                    * 
 *===========================================================================*/
PRIVATE int try_async(caller_ptr)
struct proc *caller_ptr;
{
        int r;
        struct priv *privp;
        struct proc *src_ptr;
        int postponed = FALSE;

        /* Try all privilege structures */
        for (privp = BEG_PRIV_ADDR; privp < END_PRIV_ADDR; ++privp) 
        {
                if (privp->s_proc_nr == NONE)
                        continue;

                src_ptr= proc_addr(privp->s_proc_nr);

                vmassert(!(caller_ptr->p_misc_flags & MF_DELIVERMSG));
                r= try_one(src_ptr, caller_ptr, &postponed);
                if (r == OK)
                        return r;
        }

        /* Nothing found, clear MF_ASYNMSG unless messages were postponed */
        if (postponed == FALSE)
                caller_ptr->p_misc_flags &= ~MF_ASYNMSG;

        return ESRCH;
}


/*===========================================================================*
 *                              try_one                                      *
 *===========================================================================*/
PRIVATE int try_one(src_ptr, dst_ptr, postponed)
struct proc *src_ptr;
struct proc *dst_ptr;
int *postponed;
{
        int i, do_notify, done;
        unsigned flags;
        size_t size;
        endpoint_t dst_e;
        asynmsg_t *table_ptr;
        message *m_ptr;
        struct priv *privp;
        asynmsg_t tabent;
        vir_bytes table_v;
        struct proc *caller_ptr;
        int r;

        privp= priv(src_ptr);

        /* Basic validity checks */
        if (privp->s_id == USER_PRIV_ID) return EAGAIN;
        if (privp->s_asynsize == 0) return EAGAIN;
        if (!may_send_to(src_ptr, proc_nr(dst_ptr))) return EAGAIN;

        size= privp->s_asynsize;
        table_v = privp->s_asyntab;
        caller_ptr = src_ptr;

        dst_e= dst_ptr->p_endpoint;

        /* Scan the table */
        do_notify= FALSE;       
        done= TRUE;
        for (i= 0; i<size; i++)
        {
                /* Read status word */
                A_RETRIEVE(i, flags);
                flags= tabent.flags;

                /* Skip empty entries */
                if (flags == 0)
                {
                        continue;
                }

                /* Check for reserved bits in the flags field */
                if (flags & ~(AMF_VALID|AMF_DONE|AMF_NOTIFY|AMF_NOREPLY) ||
                        !(flags & AMF_VALID))
                {
                        kprintf("try_one: bad bits in table\n");
                        privp->s_asynsize= 0;
                        return EINVAL;
                }

                /* Skip entry is AMF_DONE is already set */
                if (flags & AMF_DONE)
                {
                        continue;
                }

                /* Clear done. We are done when all entries are either empty
                 * or done at the start of the call.
                 */
                done= FALSE;

                /* Get destination */
                A_RETRIEVE(i, dst);

                if (tabent.dst != dst_e)
                {
                        continue;
                }

                /* If AMF_NOREPLY is set, do not satisfy the receiving part of
                 * a SENDREC. Do not unset MF_ASYNMSG later because of this,
                 * though: this message is still to be delivered later.
                 */
                if ((flags & AMF_NOREPLY) &&
                        (dst_ptr->p_misc_flags & MF_REPLY_PEND))
                {
                        if (postponed != NULL)
                                *postponed = TRUE;

                        continue;
                }

                /* Deliver message */
                table_ptr= (asynmsg_t *)privp->s_asyntab;
                m_ptr= &table_ptr[i].msg;       /* Note: pointer in the
                                                 * caller's address space.
                                                 */
                A_RETRIEVE(i, msg);
                r = QueueMess(src_ptr->p_endpoint, vir2phys(&tabent.msg),
                        dst_ptr);

                tabent.result= r;
                A_INSERT(i, result);
                tabent.flags= flags | AMF_DONE;
                A_INSERT(i, flags);

                if (flags & AMF_NOTIFY)
                {
                        kprintf("try_one: should notify caller\n");
                }
                return OK;
        }
        if (done)
                privp->s_asynsize= 0;
        return EAGAIN;
}

 /*===========================================================================*
 *                              lock_notify                                  *
 *===========================================================================*/
PUBLIC int lock_notify(src_e, dst_e)
int src_e;                      /* (endpoint) sender of the notification */
int dst_e;                      /* (endpoint) 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, src_p;

  vmassert(!intr_disabled());

  if (!isokendpt(src_e, &src_p)) {
        kprintf("lock_notify: bogus src: %d\n", src_e);
        return EDEADSRCDST;
  }

      lock;
  vmassert(intr_disabled());
      result = mini_notify(proc_addr(src_p), dst_e); 
  vmassert(intr_disabled());
      unlock;
  vmassert(!intr_disabled());

  return(result);
}

/*===========================================================================*
 *                              enqueue                                      * 
 *===========================================================================*/
PUBLIC 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 */

  NOREC_ENTER(enqueuefunc);

#if DEBUG_SCHED_CHECK
  if(!intr_disabled()) { minix_panic("enqueue with interrupts enabled", NO_NUM); }
  if (rp->p_ready) minix_panic("enqueue already ready process", NO_NUM);
#endif

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

  vmassert(q >= 0);
  vmassert(q < IDLE_Q || rp->p_endpoint == IDLE);

  /* 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 */
  }

#if DEBUG_SCHED_CHECK
  rp->p_ready = 1;
  CHECK_RUNQUEUES;
#endif

  /* Now select the next process to run, if there isn't a current
   * process yet or current process isn't ready any more, or
   * it's PREEMPTIBLE.
   */
        vmassert(proc_ptr);
  if((proc_ptr->p_priority > rp->p_priority) &&
   (priv(proc_ptr)->s_flags & PREEMPTIBLE)) 
     pick_proc();

#if DEBUG_SCHED_CHECK
  CHECK_RUNQUEUES;
#endif

  NOREC_RETURN(enqueuefunc, );
}

/*===========================================================================*
 *                              dequeue                                      * 
 *===========================================================================*/
PUBLIC 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;

  NOREC_ENTER(dequeuefunc);

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

#if DEBUG_SCHED_CHECK
  if(!intr_disabled()) { minix_panic("dequeue with interrupts enabled", NO_NUM); }
  if (! rp->p_ready) minix_panic("dequeue() already unready process", NO_NUM);
#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 DEBUG_SCHED_CHECK
                rp->p_ready = 0;
                  CHECK_RUNQUEUES;
#endif
          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
  CHECK_RUNQUEUES;
#endif

  NOREC_RETURN(dequeuefunc, );
}

/*===========================================================================*
 *                              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.  
 */
  int time_left = (rp->p_ticks_left > 0);       /* quantum fully consumed */

  /* Check whether the process has time left. Otherwise give a new quantum 
   * and lower the process' priority, unless the process already is in the 
   * lowest queue.  
   */
  if (! time_left) {                            /* quantum consumed ? */
      rp->p_ticks_left = rp->p_quantum_size;    /* give new quantum */
      if (rp->p_priority < (IDLE_Q-1)) {         
          rp->p_priority += 1;                  /* lower priority */
      }
  }

  /* 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 */

  NOREC_ENTER(pick);

  /* 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++) { 
        int found = 0;
        if(!(rp = rdy_head[q])) {
                TRACE(VF_PICKPROC, printf("queue %d empty\n", q););
                continue;
        }
        TRACE(VF_PICKPROC, printf("found %s / %d on queue %d\n", 
                rp->p_name, rp->p_endpoint, q););
        next_ptr = rp;                  /* run process 'rp' next */
        vmassert(proc_ptr != next_ptr);
        vmassert(!next_ptr->p_rts_flags);
        if (priv(rp)->s_flags & BILLABLE)               
                bill_ptr = rp;          /* bill for system time */
        NOREC_RETURN(pick, );
  }
}

/*===========================================================================*
 *                              balance_queues                               *
 *===========================================================================*/
#define Q_BALANCE_TICKS  100
PUBLIC void balance_queues(tp)
timer_t *tp;                                    /* watchdog timer pointer */
{
/* Check entire process table and give all process a higher priority. This
 * effectively means giving a new quantum. If a process already is at its 
 * maximum priority, its quantum will be renewed.
 */
  static timer_t queue_timer;                   /* timer structure to use */
  register struct proc* rp;                     /* process table pointer  */
  clock_t next_period;                          /* time of next period  */
  int ticks_added = 0;                          /* total time added */

  vmassert(!intr_disabled());

  lock;
  for (rp=BEG_PROC_ADDR; rp<END_PROC_ADDR; rp++) {
      if (! isemptyp(rp)) {                             /* check slot use */
          if (rp->p_priority > rp->p_max_priority) {    /* update priority? */
              if (rp->p_rts_flags == 0) dequeue(rp);    /* take off queue */
              ticks_added += rp->p_quantum_size;        /* do accounting */
              rp->p_priority -= 1;                      /* raise priority */
              if (rp->p_rts_flags == 0) enqueue(rp);    /* put on queue */
          }
          else {
              ticks_added += rp->p_quantum_size - rp->p_ticks_left;
              rp->p_ticks_left = rp->p_quantum_size;    /* give new quantum */
          }
      }
  }
  unlock;

  /* Now schedule a new watchdog timer to balance the queues again.  The 
   * period depends on the total amount of quantum ticks added.
   */
  next_period = MAX(Q_BALANCE_TICKS, ticks_added);      /* calculate next */
  set_timer(&queue_timer, get_uptime() + next_period, balance_queues);
}

/*===========================================================================*
 *                              lock_send                                    *
 *===========================================================================*/
PUBLIC int lock_send(dst_e, m_ptr)
int dst_e;                      /* to whom is message being sent? */
message *m_ptr;                 /* pointer to message buffer */
{
/* Safe gateway to mini_send() for tasks. */
  int result;
  lock;
  result = mini_send(proc_ptr, dst_e, m_ptr, 0);
  unlock;
  return(result);
}

/*===========================================================================*
 *                              endpoint_lookup                              *
 *===========================================================================*/
PUBLIC struct proc *endpoint_lookup(endpoint_t e)
{
        int n;

        if(!isokendpt(e, &n)) return NULL;

        return proc_addr(n);
}

/*===========================================================================*
 *                              isokendpt_f                                  *
 *===========================================================================*/
#if DEBUG_ENABLE_IPC_WARNINGS
PUBLIC int isokendpt_f(file, line, e, p, fatalflag)
char *file;
int line;
#else
PUBLIC int isokendpt_f(e, p, fatalflag)
#endif
endpoint_t e;
int *p, fatalflag;
{
        int ok = 0;
        /* Convert an endpoint number into a process number.
         * Return nonzero if the process is alive with the corresponding
         * generation number, zero otherwise.
         *
         * This function is called with file and line number by the
         * isokendpt_d macro if DEBUG_ENABLE_IPC_WARNINGS is defined,
         * otherwise without. This allows us to print the where the
         * conversion was attempted, making the errors verbose without
         * adding code for that at every call.
         * 
         * If fatalflag is nonzero, we must panic if the conversion doesn't
         * succeed.
         */
        *p = _ENDPOINT_P(e);
        if(!isokprocn(*p)) {
#if DEBUG_ENABLE_IPC_WARNINGS
                kprintf("kernel:%s:%d: bad endpoint %d: proc %d out of range\n",
                file, line, e, *p);
#endif
        } else if(isemptyn(*p)) {
#if 0
        kprintf("kernel:%s:%d: bad endpoint %d: proc %d empty\n", file, line, e, *p);
#endif
        } else if(proc_addr(*p)->p_endpoint != e) {
#if DEBUG_ENABLE_IPC_WARNINGS
                kprintf("kernel:%s:%d: bad endpoint %d: proc %d has ept %d (generation %d vs. %d)\n", file, line,
                e, *p, proc_addr(*p)->p_endpoint,
                _ENDPOINT_G(e), _ENDPOINT_G(proc_addr(*p)->p_endpoint));
#endif
        } else ok = 1;
        if(!ok && fatalflag) {
                minix_panic("invalid endpoint ", e);
        }
        return ok;
}