sched_switch (sched_4bsd) may be preempted

[Stephan Uphoff]

sched_switch (sched_4bsd) may be preempted in setrunqueue or slot_fill.
This could be ugly.
Wrapping it into a critical section and resetting TDP_OWEPREEMPT should
work.

Hand trimmed patch:

RCS file: /cvsroot/src/sys/kern/sched_4bsd.c,v
retrieving revision 1.65
diff -u -r1.65 sched_4bsd.c
--- sys/kern/sched_4bsd.c       16 Sep 2004 07:12:59 -0000      1.65
+++ sys/kern/sched_4bsd.c       1 Oct 2004 05:35:28 -0000
@@ -823,6 +823,7 @@
                TD_SET_CAN_RUN(td);
        else {
                td->td_ksegrp->kg_avail_opennings++;
+               critical_enter();
                if (TD_IS_RUNNING(td)) {
                        /* Put us back on the run queue (kse and all).
*/
                        setrunqueue(td, SRQ_OURSELF|SRQ_YIELDING);
@@ -834,6 +835,8 @@
                         */
                        slot_fill(td->td_ksegrp);
                }
+               critical_exit();
+               td->td_pflags &= ~TDP_OWEPREEMPT;
        }
        if (newtd == NULL)
                newtd = choosethread();