domctl: improve locking during domain destruction

There is no need to hold the global domctl lock across domain_kill() -
the domain lock is fully sufficient here, and parallel cleanup after
multiple domains performs quite a bit better this way.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/domain.c

@@ -615,13 +615,21 @@ int domain_kill(struct domain *d)
     if ( d == current->domain )
         return -EINVAL;
 
-    /* Protected by domctl_lock. */
+    /* Protected by d->domain_lock. */
     switch ( d->is_dying )
     {
     case DOMDYING_alive:
+        domain_unlock(d);
         domain_pause(d);
+        domain_lock(d);
+        /*
+         * With the domain lock dropped, d->is_dying may have changed. Call
+         * ourselves recursively if so, which is safe as then we won't come
+         * back here.
+         */
+        if ( d->is_dying != DOMDYING_alive )
+            return domain_kill(d);
         d->is_dying = DOMDYING_dying;
-        spin_barrier(&d->domain_lock);
         evtchn_destroy(d);
         gnttab_release_mappings(d);
         tmem_destroy(d->tmem_client);

xen/common/domctl.c

@@ -665,11 +665,14 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
         break;
 
     case XEN_DOMCTL_destroydomain:
+        domctl_lock_release();
+        domain_lock(d);
         ret = domain_kill(d);
+        domain_unlock(d);
         if ( ret == -ERESTART )
             ret = hypercall_create_continuation(
                 __HYPERVISOR_domctl, "h", u_domctl);
-        break;
+        goto domctl_out_unlock_domonly;
 
     case XEN_DOMCTL_setnodeaffinity:
     {