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[Qemu-devel] [RFC V5 28/36] qcow2: Do not overwrite existing entries wit


From: Benoît Canet
Subject: [Qemu-devel] [RFC V5 28/36] qcow2: Do not overwrite existing entries with QCOW_OFLAG_COPIED.
Date: Wed, 16 Jan 2013 17:24:49 +0100

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 <address@hidden>
---
 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




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