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[Qemu-devel] [PATCH] block: fix external snapshot abort permission error

From: Jeff Cody
Subject: [Qemu-devel] [PATCH] block: fix external snapshot abort permission error
Date: Wed, 7 Jun 2017 09:55:22 -0400

In external_snapshot_abort(), we try to undo what was done in
external_snapshot_prepare() calling bdrv_replace_node() to swap the
nodes back.  However, we receive a permissions error as writers are
blocked on the old node, which is now the new node backing file.

An easy fix (initially suggested by Kevin Wolf) is to call
bdrv_set_backing_hd() on the new node, to set the backing node to NULL.

Signed-off-by: Jeff Cody <address@hidden>
 blockdev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/blockdev.c b/blockdev.c
index 892d768..6472548 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1803,7 +1803,11 @@ static void external_snapshot_abort(BlkActionState 
                              DO_UPCAST(ExternalSnapshotState, common, common);
     if (state->new_bs) {
         if (state->overlay_appended) {
+            bdrv_ref(state->old_bs);   /* we can't let bdrv_set_backind_hd()
+                                          close state->old_bs; we need it */
+            bdrv_set_backing_hd(state->new_bs, NULL, &error_abort);
             bdrv_replace_node(state->new_bs, state->old_bs, &error_abort);
+            bdrv_unref(state->old_bs); /* bdrv_replace_node() ref'ed old_bs */

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