block: Add bdrv_co_enter()/leave()

Add a pair of functions to temporarily move the current coroutine to the
AioContext of a given BlockDriverState.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201005155855.256490-13-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/include/block/block.h b/include/block/block.h
index ce2ac39..1027c58 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -641,6 +641,23 @@ bool bdrv_debug_is_suspended(BlockDriverState *bs, const char *tag);
 AioContext *bdrv_get_aio_context(BlockDriverState *bs);
 
 /**
+ * Move the current coroutine to the AioContext of @bs and return the old
+ * AioContext of the coroutine. Increase bs->in_flight so that draining @bs
+ * will wait for the operation to proceed until the corresponding
+ * bdrv_co_leave().
+ *
+ * Consequently, you can't call drain inside a bdrv_co_enter/leave() section as
+ * this will deadlock.
+ */
+AioContext *coroutine_fn bdrv_co_enter(BlockDriverState *bs);
+
+/**
+ * Ends a section started by bdrv_co_enter(). Move the current coroutine back
+ * to old_ctx and decrease bs->in_flight again.
+ */
+void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx);
+
+/**
  * Transfer control to @co in the aio context of @bs
  */
 void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co);