block: Introduce BlockDriver.bdrv_drain callback

Drivers can have internal request sources that generate IO, like the
need_check_timer in QED. Since we want quiesced periods that contain
nested event loops in block layer, we need to have a way to disable such
event sources.

Block drivers must implement the "bdrv_drain" callback if it has any
internal sources that can generate I/O activity, like a timer or a
worker thread (even in a library) that can schedule QEMUBH in an
asynchronous callback.

Update the comments of bdrv_drain and bdrv_drained_begin accordingly.

Like bdrv_requests_pending(), we should consider all the children of bs.
Before, the while loop just works, as bdrv_requests_pending() already
tracks its children; now we mustn't miss the callback, so recurse down
explicitly.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1447064214-29930-9-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -237,8 +237,21 @@ bool bdrv_requests_pending(BlockDriverState *bs)
     return false;
 }
 
+static void bdrv_drain_recurse(BlockDriverState *bs)
+{
+    BdrvChild *child;
+
+    if (bs->drv && bs->drv->bdrv_drain) {
+        bs->drv->bdrv_drain(bs);
+    }
+    QLIST_FOREACH(child, &bs->children, next) {
+        bdrv_drain_recurse(child->bs);
+    }
+}
+
 /*
- * Wait for pending requests to complete on a single BlockDriverState subtree
+ * Wait for pending requests to complete on a single BlockDriverState subtree,
+ * and suspend block driver's internal I/O until next request arrives.
  *
  * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
  * AioContext.
@@ -251,6 +264,7 @@ void bdrv_drain(BlockDriverState *bs)
 {
     bool busy = true;
 
+    bdrv_drain_recurse(bs);
     while (busy) {
         /* Keep iterating */
          bdrv_flush_io_queue(bs);

include/block/block_int.h

@@ -295,6 +295,12 @@ struct BlockDriver {
      */
     int (*bdrv_probe_geometry)(BlockDriverState *bs, HDGeometry *geo);
 
+    /**
+     * Drain and stop any internal sources of requests in the driver, and
+     * remain so until next I/O callback (e.g. bdrv_co_writev) is called.
+     */
+    void (*bdrv_drain)(BlockDriverState *bs);
+
     QLIST_ENTRY(BlockDriver) list;
 };