Revert "domctl: improve locking during domain destruction"

This reverts commit 228ab9992f.

Performance analysis has shown that dropping the domctl lock during
domain destruction greatly increases the contention in the heap_lock,
thus making parallel destruction of domains slower.

The following lockperf data shows the difference between the current
code and the reverted one:

lock:  3342357(2.268295505s), block:  3263853(18.556650797s)
lock:  2788704(0.362311723s), block:   222681( 0.091152276s)

Those figures are from Dmitry Isaikin, and are gathered after
destroying 5 2GB HVM guests in parallel:

https://lists.xenproject.org/archives/html/xen-devel/2021-09/msg01515.html

Given the current point in the release, revert the commit and
reinstate holding the domctl lock during domain destruction. Further
work should be done in order to re-add more fine grained locking to
the domain destruction path once a proper solution to avoid the
heap_lock contention is found.

Reported-by: Hongyan Xia <hongyxia@amazon.com>
Reported-by: Dmitry Isaikin <isaikin-dmitry@yandex.ru>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/common/domain.c

@@ -917,21 +917,13 @@ int domain_kill(struct domain *d)
     if ( d == current->domain )
         return -EINVAL;
 
-    /* Protected by d->domain_lock. */
+    /* Protected by domctl_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);
         argo_destroy(d);
         vnuma_destroy(d->vnuma);
         domain_set_outstanding_pages(d, 0);

xen/common/domctl.c

@@ -497,14 +497,11 @@ 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);
-        goto domctl_out_unlock_domonly;
+        break;
 
     case XEN_DOMCTL_setnodeaffinity:
     {