kernel - Fix long-standing vfork/exec bug

* Fix an issue where the parent process can get stuck in the "ppwait"
  state due to a tsleep/wakeup race with its child.

* This bug was present but masked by other tokens prior to a few patches
  ago.  With those tokens now gone this bug reared its face.

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 086c2cd..d563a53 100644 (file)
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -745,8 +745,12 @@ start_forked_proc(struct lwp *lp1, struct proc *p2)
        /*
         * Preserve synchronization semantics of vfork.  If waiting for
         * child to exec or exit, set P_PPWAIT on child, and sleep on our
-        * proc (in case of exit).
+        * proc (in case of exec or exit).
+        *
+        * We must hold our p_token to interlock the flag/tsleep
         */
+       lwkt_gettoken(&p2->p_token);
        while (p2->p_flag & P_PPWAIT)
                tsleep(lp1->lwp_proc, 0, "ppwait", 0);
+       lwkt_reltoken(&p2->p_token);
 }