From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49091) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Z2gM3-0005Db-J0 for qemu-devel@nongnu.org; Wed, 10 Jun 2015 09:47:44 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Z2gM2-0007n3-5R for qemu-devel@nongnu.org; Wed, 10 Jun 2015 09:47:43 -0400 From: Kevin Wolf Date: Wed, 10 Jun 2015 15:47:03 +0200 Message-Id: <1433944027-28533-10-git-send-email-kwolf@redhat.com> In-Reply-To: <1433944027-28533-1-git-send-email-kwolf@redhat.com> References: <1433944027-28533-1-git-send-email-kwolf@redhat.com> Subject: [Qemu-devel] [PATCH v2 09/13] block: Drain requests before swapping nodes in bdrv_swap() List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-block@nongnu.org Cc: kwolf@redhat.com, berto@igalia.com, qemu-devel@nongnu.org, armbru@redhat.com, mreitz@redhat.com bdrv_swap() requires that there are no requests in flight on either of the two devices. The request coroutine would work on the wrong BlockDriverState object (with bs->opaque even being interpreted as a different type potentially) and all sorts of bad things would result from this. The currently existing callers mostly ensure that there is no I/O pending on nodes that are swapped. In detail, this is: 1. Live snapshots. This goes through qmp_transaction(), which calls bdrv_drain_all() before doing anything. The command is executed synchronously, so no new I/O can be issued concurrently. 2. snapshot=on in bdrv_open(). We're in the middle of opening the image (both the original image and its temporary overlay), so there can't be any I/O in flight yet. 3. Mirroring. bdrv_drain() is already used on the source device so that the mirror doesn't miss anything. However, the main loop runs between that and the bdrv_swap() (which is actually a bug, being addressed in another series), so there is a small window in which new I/O might be issued that would be in flight during bdrv_swap(). It is safer to just drain the request queue of both devices in bdrv_swap() instead of relying on callers to do the right thing. Signed-off-by: Kevin Wolf --- block.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/block.c b/block.c index 7ba0edd..f1ceb26 100644 --- a/block.c +++ b/block.c @@ -1947,6 +1947,9 @@ void bdrv_swap(BlockDriverState *bs_new, BlockDriverState *bs_old) { BlockDriverState tmp; + bdrv_drain(bs_new); + bdrv_drain(bs_old); + /* The code needs to swap the node_name but simply swapping node_list won't * work so first remove the nodes from the graph list, do the swap then * insert them back if needed. @@ -1990,6 +1993,9 @@ void bdrv_swap(BlockDriverState *bs_new, BlockDriverState *bs_old) QTAILQ_INSERT_TAIL(&graph_bdrv_states, bs_old, node_list); } + assert(QLIST_EMPTY(&bs_old->tracked_requests)); + assert(QLIST_EMPTY(&bs_new->tracked_requests)); + bdrv_rebind(bs_new); bdrv_rebind(bs_old); } -- 1.8.3.1