docs: mark nested AioContext locking as a legacy API
See the patch for why nested AioContext locking is no longer allowed.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171207201320.19284-3-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/docs/devel/multiple-iothreads.txt b/docs/devel/multiple-iothreads.txt
index e4d340b..4f9012d 100644
--- a/docs/devel/multiple-iothreads.txt
+++ b/docs/devel/multiple-iothreads.txt
@@ -1,4 +1,4 @@
-Copyright (c) 2014 Red Hat Inc.
+Copyright (c) 2014-2017 Red Hat Inc.
This work is licensed under the terms of the GNU GPL, version 2 or later. See
the COPYING file in the top-level directory.
@@ -92,8 +92,9 @@
context is acquired no other thread can access it or run event loop iterations
in this AioContext.
-aio_context_acquire()/aio_context_release() calls may be nested. This
-means you can call them if you're not sure whether #2 applies.
+Legacy code sometimes nests aio_context_acquire()/aio_context_release() calls.
+Do not use nesting anymore, it is incompatible with the BDRV_POLL_WHILE() macro
+used in the block layer and can lead to hangs.
There is currently no lock ordering rule if a thread needs to acquire multiple
AioContexts simultaneously. Therefore, it is only safe for code holding the
