after enqueue()ing a process, only pick_proc() a new one if the current

process is not PREEMPTIBLE (or it's not ready, or there isn't a current
process yet). This fixes a case where a process that isn't
PREEMPTIBLE actually gets preempted. (This solves a race condition
between CLOCK and SYSTEM.)

diff --git a/kernel/proc.c b/kernel/proc.c
index dede6f9e4f479f939cac70f8b254a19e65560726..31de06de7f096a55ef6b4536be28f86fe7e77d4b 100755 (executable)
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -523,8 +523,14 @@ register struct proc *rp;  /* this process is now runnable */
       rp->p_nextready = NIL_PROC;              /* mark new end */
   }
 
-  /* Now select the next process to run. */
-  pick_proc();                 
+  /* 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.
+   */
+  if(!proc_ptr || proc_ptr->p_rts_flags ||
+    (priv(proc_ptr)->s_flags & PREEMPTIBLE)) {
+     pick_proc();
+  }
 
 #if DEBUG_SCHED_CHECK
   rp->p_ready = 1;
@@ -554,8 +560,7 @@ register struct proc *rp;   /* this process is no longer runnable */
 
 #if DEBUG_SCHED_CHECK
   check_runqueues("dequeue1");
-  if (! rp->p_ready) kprintf("%s:%d: dequeue() already unready process\n",
-       f_str, f_line);
+  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 
@@ -639,6 +644,7 @@ PRIVATE void pick_proc()
           return;                               
       }
   }
+  panic("no ready process", NO_NUM);
 }
 
 /*===========================================================================*