[Xen-devel] [PATCH RFC] domctl: improve locking during domain destruction

["Jan Beulich" <JBeulich@xxxxxxxx>]

There is no need to hold the global domctl lock while across
domain_kill() - the domain lock is fully sufficient here.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Sadly so far we haven't had any feedback on this change from the people
who observed an issue apparently resulting from heavy contention here.
Hence the RFC.

Obviously other domctl-s could benefit from similar adjustments, so
this is meant to be just a start.

--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -619,13 +619,16 @@ 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);
+        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);
--- a/xen/common/domctl.c
+++ b/xen/common/domctl.c
@@ -664,11 +664,14 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xe
         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:
     {




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