xen-block: Fix double qlist remove and request leak

Commit a31ca6801c02 ("qemu/queue.h: clear linked list pointers on
remove") revealed that a request was removed twice from a list, once
in xen_block_finish_request() and a second time in
xen_block_release_request() when both function are called from
xen_block_complete_aio(). But also, the `requests_inflight' counter is
decreased twice, and thus became negative.

This is a bug that was introduced in bfd0d6366043 ("xen-block: improve
response latency"), where a `finished' list was removed.

That commit also introduced a leak of request in xen_block_do_aio().
That function calls xen_block_finish_request() but the request is
never released after that.

To fix both issue, we do two changes:
- we squash finish_request() and release_request() together as we want
  to remove a request from 'inflight' list to add it to 'freelist'.
- before releasing a request, we need to let the other end know the
  result, thus we should call xen_block_send_response() before
  releasing a request.

The first change fixes the double QLIST_REMOVE() as we remove the extra
call. The second change makes the leak go away because if we want to
call finish_request(), we need to call a function that does all of
finish, send response, and release.

Fixes: bfd0d6366043 ("xen-block: improve response latency")
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200406140217.1441858-1-anthony.perard@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
[mreitz: Amended commit message as per Paul's suggestions]
Signed-off-by: Max Reitz <mreitz@redhat.com>

hw/block/dataplane/xen-block.c

@@ -64,6 +64,8 @@ struct XenBlockDataPlane {
     AioContext *ctx;
 };
 
+static int xen_block_send_response(XenBlockRequest *request);
+
 static void reset_request(XenBlockRequest *request)
 {
     memset(&request->req, 0, sizeof(request->req));
@@ -115,23 +117,26 @@ out:
     return request;
 }
 
-static void xen_block_finish_request(XenBlockRequest *request)
+static void xen_block_complete_request(XenBlockRequest *request)
 {
     XenBlockDataPlane *dataplane = request->dataplane;
 
+    if (xen_block_send_response(request)) {
+        Error *local_err = NULL;
+
+        xen_device_notify_event_channel(dataplane->xendev,
+                                        dataplane->event_channel,
+                                        &local_err);
+        if (local_err) {
+            error_report_err(local_err);
+        }
+    }
+
     QLIST_REMOVE(request, list);
     dataplane->requests_inflight--;
-}
-
-static void xen_block_release_request(XenBlockRequest *request)
-{
-    XenBlockDataPlane *dataplane = request->dataplane;
-
-    QLIST_REMOVE(request, list);
     reset_request(request);
     request->dataplane = dataplane;
     QLIST_INSERT_HEAD(&dataplane->freelist, request, list);
-    dataplane->requests_inflight--;
 }
 
 /*
@@ -246,7 +251,6 @@ static int xen_block_copy_request(XenBlockRequest *request)
 }
 
 static int xen_block_do_aio(XenBlockRequest *request);
-static int xen_block_send_response(XenBlockRequest *request);
 
 static void xen_block_complete_aio(void *opaque, int ret)
 {
@@ -286,7 +290,6 @@ static void xen_block_complete_aio(void *opaque, int ret)
     }
 
     request->status = request->aio_errors ? BLKIF_RSP_ERROR : BLKIF_RSP_OKAY;
-    xen_block_finish_request(request);
 
     switch (request->req.operation) {
     case BLKIF_OP_WRITE:
@@ -306,17 +309,8 @@ static void xen_block_complete_aio(void *opaque, int ret)
     default:
         break;
     }
-    if (xen_block_send_response(request)) {
-        Error *local_err = NULL;
 
-        xen_device_notify_event_channel(dataplane->xendev,
+    xen_block_complete_request(request);
-                                        dataplane->event_channel,
-                                        &local_err);
-        if (local_err) {
-            error_report_err(local_err);
-        }
-    }
-    xen_block_release_request(request);
 
     if (dataplane->more_work) {
         qemu_bh_schedule(dataplane->bh);
@@ -420,8 +414,8 @@ static int xen_block_do_aio(XenBlockRequest *request)
     return 0;
 
 err:
-    xen_block_finish_request(request);
     request->status = BLKIF_RSP_ERROR;
+    xen_block_complete_request(request);
     return -1;
 }
 
@@ -575,17 +569,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
                 break;
             };
 
-            if (xen_block_send_response(request)) {
+            xen_block_complete_request(request);
-                Error *local_err = NULL;
-
-                xen_device_notify_event_channel(dataplane->xendev,
-                                                dataplane->event_channel,
-                                                &local_err);
-                if (local_err) {
-                    error_report_err(local_err);
-                }
-            }
-            xen_block_release_request(request);
             continue;
         }