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 <ben...@irqsave.net> --- block/qcow2-cluster.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c index fedcf57..c016e85 100644 --- a/block/qcow2-cluster.c +++ b/block/qcow2-cluster.c @@ -763,6 +763,11 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m) for (i = 0; i < m->nb_clusters; i++) { uint64_t flags = 0; uint64_t offset = cluster_offset + (i << s->cluster_bits); + + if (be64_to_cpu(l2_table[l2_index + i]) == offset) { + 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