[PATCH] x86/mem_sharing: don't lock parent during fork reset

[Tamas K Lengyel <tamas.lengyel@intel.com>]

During fork reset operation the parent domain doesn't need to be gathered using
rcu_lock_live_remote_domain_by_id as the fork reset doesn't modify anything on
the parent. The parent is also guaranteed to be paused while forks are active.
This patch reduces lock contention when performing resets in parallel.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@intel.com>
---
 xen/arch/x86/mm/mem_sharing.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 8d5d44bdbc..b80b978ef3 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -1879,9 +1879,10 @@ static int fork(struct domain *cd, struct domain *d)
  * footprints the hypercall continuation should be implemented (or if this
  * feature needs to be become "stable").
  */
-static int mem_sharing_fork_reset(struct domain *d, struct domain *pd)
+static int mem_sharing_fork_reset(struct domain *d)
 {
     int rc;
+    struct domain *pd = d->parent;
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
     struct page_info *page, *tmp;
 
@@ -2226,8 +2227,6 @@ int mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
 
     case XENMEM_sharing_op_fork_reset:
     {
-        struct domain *pd;
-
         rc = -EINVAL;
         if ( mso.u.fork.pad || mso.u.fork.flags )
             goto out;
@@ -2236,13 +2235,7 @@ int mem_sharing_memop(XEN_GUEST_HANDLE_PARAM(xen_mem_sharing_op_t) arg)
         if ( !d->parent )
             goto out;
 
-        rc = rcu_lock_live_remote_domain_by_id(d->parent->domain_id, &pd);
-        if ( rc )
-            goto out;
-
-        rc = mem_sharing_fork_reset(d, pd);
-
-        rcu_unlock_domain(pd);
+        rc = mem_sharing_fork_reset(d);
         break;
     }
 
-- 
2.25.1