From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46112) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eN2Y3-0007O0-Ey for qemu-devel@nongnu.org; Thu, 07 Dec 2017 15:13:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eN2Y2-0006iK-IJ for qemu-devel@nongnu.org; Thu, 07 Dec 2017 15:13:35 -0500 From: Stefan Hajnoczi Date: Thu, 7 Dec 2017 20:13:16 +0000 Message-Id: <20171207201320.19284-3-stefanha@redhat.com> In-Reply-To: <20171207201320.19284-1-stefanha@redhat.com> References: <20171207201320.19284-1-stefanha@redhat.com> Subject: [Qemu-devel] [PATCH v2 2/6] docs: mark nested AioContext locking as a legacy API List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: qemu-block@nongnu.org, Eric Blake , Kevin Wolf , Paolo Bonzini , Stefan Hajnoczi See the patch for why nested AioContext locking is no longer allowed. Signed-off-by: Stefan Hajnoczi --- docs/devel/multiple-iothreads.txt | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/docs/devel/multiple-iothreads.txt b/docs/devel/multiple-iothreads.txt index e4d340bbb7..4f9012d154 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 @@ aio_context_acquire()/aio_context_release() for mutual exclusion. Once the 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 -- 2.14.3