block: Add bdrv_lock()/unlock()

Inside of coroutine context, we can't directly use aio_context_acquire()
for the AioContext of a block node because we already own the lock of
the current AioContext and we need to avoid double locking to prevent
deadlocks.

This provides helper functions to lock the AioContext of a node only if
it's not the same as the current AioContext.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201005155855.256490-14-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/block/block.h b/include/block/block.h
index 1027c58a41..d16c401cb4 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -658,6 +658,20 @@ AioContext *coroutine_fn bdrv_co_enter(BlockDriverState *bs);
 void coroutine_fn bdrv_co_leave(BlockDriverState *bs, AioContext *old_ctx);
 
 /**
+ * Locks the AioContext of @bs if it's not the current AioContext. This avoids
+ * double locking which could lead to deadlocks: This is a coroutine_fn, so we
+ * know we already own the lock of the current AioContext.
+ *
+ * May only be called in the main thread.
+ */
+void coroutine_fn bdrv_co_lock(BlockDriverState *bs);
+
+/**
+ * Unlocks the AioContext of @bs if it's not the current AioContext.
+ */
+void coroutine_fn bdrv_co_unlock(BlockDriverState *bs);
+
+/**
  * Transfer control to @co in the aio context of @bs
  */
 void bdrv_coroutine_enter(BlockDriverState *bs, Coroutine *co);