[Qemu-devel] [RFC V3 22/24] qcow2: Do not overwrite existing entries with QCOW_OFLAG_COPIED.

["Benoît Canet" <benoit@irqsave.net>]

In the case of a race condition between two writes a l2 entry can be written
without QCOW_OFLAG_COPIED before the first write fill it.
This patch simply check if the l2 entry has the correct offset without
QCOW_OFLAG_COPIED and do nothing.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
---
 block/qcow2-cluster.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 8db1b2a..0042742 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -699,6 +699,10 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
 
     for (i = 0; i < m->nb_clusters; i++) {
+        if (be64_to_cpu(l2_table[l2_index + i]) ==
+            (cluster_offset + (i << s->cluster_bits))) {
+            continue;
+        }
         /* if two concurrent writes happen to the same unallocated cluster
 	 * each write allocates separate cluster and writes data concurrently.
 	 * The first one to complete updates l2 table with pointer to its
-- 
1.7.10.4