[PATCH v2 2/2] Send gnttab unmap notify event after the grant is fully released

[Rafał Wojdyła <omeg@xxxxxxxxxxxxxxxxxxxxxx>]

Move the notification event to after the unmap hypercall completes,
so the peer only gets notified after the grant is completely released
on our side. Closes possible race when the peer could be notified
before the grant was fully torn down.

Signed-off-by: Rafał Wojdyła <omeg@xxxxxxxxxxxxxxxxxxxxxx>
---
 src/xeniface/ioctl_gnttab.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/src/xeniface/ioctl_gnttab.c b/src/xeniface/ioctl_gnttab.c
index ef77295..ee319cb 100644
--- a/src/xeniface/ioctl_gnttab.c
+++ b/src/xeniface/ioctl_gnttab.c
@@ -802,17 +802,11 @@ GnttabFreeMap(
 
     Trace("Context %p\n", Context);
 
+    // Write the notify value while the grant is still mapped.
     if (Context->Flags & XENIFACE_GNTTAB_USE_NOTIFY_OFFSET) {
         ((PCHAR)Context->KernelVa)[Context->NotifyOffset] = 0;
     }
 
-    if (Context->Flags & XENIFACE_GNTTAB_USE_NOTIFY_PORT) {
-        status = EvtchnNotify(Fdo, Context->NotifyPort, NULL);
-
-        if (!NT_SUCCESS(status)) // non-fatal, we must free memory
-            Error("failed to notify port %lu: 0x%x\n", Context->NotifyPort, 
status);
-    }
-
     // unmap from user address space
     MmUnmapLockedPages(Context->UserVa, Context->Mdl);
 
@@ -828,6 +822,14 @@ GnttabFreeMap(
 
     ASSERT(NT_SUCCESS(status));
 
+    // Notify the peer after the map is fully gone.
+    if (Context->Flags & XENIFACE_GNTTAB_USE_NOTIFY_PORT) {
+        status = EvtchnNotify(Fdo, Context->NotifyPort, NULL);
+
+        if (!NT_SUCCESS(status)) // non-fatal, we must free memory
+            Error("failed to notify port %lu: 0x%x\n", Context->NotifyPort, 
status);
+    }
+
     RtlZeroMemory(Context, sizeof(*Context));
     __FreePoolWithTag(Context, XENIFACE_POOL_TAG);
 }
-- 
2.54.0