From bd2ddd5fd42cb31de4bb1beb0b8310600227a33c Mon Sep 17 00:00:00 2001
From: Ben Gras <ben@minix3.org>
Date: Wed, 21 Mar 2007 09:45:01 +0000
Subject: [PATCH] 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.)

---
 kernel/proc.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/proc.c b/kernel/proc.c
index dede6f9e4..31de06de7 100755
--- 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);
 }
 
 /*===========================================================================*