diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 424bedd..3e877a4 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -609,12 +609,12 @@ static int write_l2_entries(BDRVQcowState *s, uint64_t *l2_table,
     return 0;
 }
 
-int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset,
-    QCowL2Meta *m)
+int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
 {
     BDRVQcowState *s = bs->opaque;
     int i, j = 0, l2_index, ret;
     uint64_t *old_cluster, start_sect, l2_offset, *l2_table;
+    uint64_t cluster_offset = m->cluster_offset;
 
     if (m->nb_clusters == 0)
         return 0;
@@ -675,16 +675,22 @@ err:
 /*
  * alloc_cluster_offset
  *
- * For a given offset of the disk image, return cluster offset in
- * qcow2 file.
- *
+ * For a given offset of the disk image, return cluster offset in qcow2 file.
  * If the offset is not found, allocate a new cluster.
  *
- * Return the cluster offset if successful,
- * Return 0, otherwise.
+ * If the cluster was already allocated, m->nb_clusters is set to 0,
+ * m->depends_on is set to NULL and the other fields in m are meaningless.
+ *
+ * If the cluster is newly allocated, m->nb_clusters is set to the number of
+ * contiguous clusters that have been allocated. This may be 0 if the request
+ * conflict with another write request in flight; in this case, m->depends_on
+ * is set and the remaining fields of m are meaningless.
  *
+ * If m->nb_clusters is non-zero, the other fields of m are valid and contain
+ * information about the first allocated cluster.
+ *
+ * Return 0 on success and -errno in error cases
  */
-
 uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
                                     uint64_t offset,
                                     int n_start, int n_end,
@@ -698,7 +704,7 @@ uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
 
     ret = get_cluster_table(bs, offset, &l2_table, &l2_offset, &l2_index);
     if (ret < 0) {
-        return 0;
+        return ret;
     }
 
     nb_clusters = size_to_clusters(s, n_end << 9);
@@ -715,6 +721,7 @@ uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
 
         cluster_offset &= ~QCOW_OFLAG_COPIED;
         m->nb_clusters = 0;
+        m->depends_on = NULL;
 
         goto out;
     }
@@ -793,10 +800,11 @@ uint64_t qcow2_alloc_cluster_offset(BlockDriverState *bs,
 
 out:
     m->nb_available = MIN(nb_clusters << (s->cluster_bits - 9), n_end);
+    m->cluster_offset = cluster_offset;
 
     *num = m->nb_available - n_start;
 
-    return cluster_offset;
+    return 0;
 }
 
 static int decompress_buffer(uint8_t *out_buf, int out_buf_size,
diff --git a/block/qcow2.c b/block/qcow2.c
index e06f4dd..773d381 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -561,7 +561,7 @@ static void qcow_aio_write_cb(void *opaque, int ret)
     acb->hd_aiocb = NULL;
 
     if (ret >= 0) {
-        ret = qcow2_alloc_cluster_link_l2(bs, acb->cluster_offset, &acb->l2meta);
+        ret = qcow2_alloc_cluster_link_l2(bs, &acb->l2meta);
     }
 
     run_dependent_requests(&acb->l2meta);
@@ -585,21 +585,23 @@ static void qcow_aio_write_cb(void *opaque, int ret)
         n_end > QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors)
         n_end = QCOW_MAX_CRYPT_CLUSTERS * s->cluster_sectors;
 
-    acb->cluster_offset = qcow2_alloc_cluster_offset(bs, acb->sector_num << 9,
-                                          index_in_cluster,
-                                          n_end, &acb->n, &acb->l2meta);
+    ret = qcow2_alloc_cluster_offset(bs, acb->sector_num << 9,
+        index_in_cluster, n_end, &acb->n, &acb->l2meta);
+    if (ret < 0) {
+        goto done;
+    }
+
+    acb->cluster_offset = acb->l2meta.cluster_offset;
 
     /* Need to wait for another request? If so, we are done for now. */
-    if (!acb->cluster_offset && acb->l2meta.depends_on != NULL) {
+    if (acb->l2meta.nb_clusters == 0 && acb->l2meta.depends_on != NULL) {
         QLIST_INSERT_HEAD(&acb->l2meta.depends_on->dependent_requests,
             acb, next_depend);
         return;
     }
 
-    if (!acb->cluster_offset || (acb->cluster_offset & 511) != 0) {
-        ret = -EIO;
-        goto done;
-    }
+    assert((acb->cluster_offset & 511) == 0);
+
     if (s->crypt_method) {
         if (!acb->cluster_data) {
             acb->cluster_data = qemu_mallocz(QCOW_MAX_CRYPT_CLUSTERS *
@@ -782,27 +784,27 @@ static int get_bits_from_size(size_t size)
 static int preallocate(BlockDriverState *bs)
 {
     BDRVQcowState *s = bs->opaque;
-    uint64_t cluster_offset = 0;
     uint64_t nb_sectors;
     uint64_t offset;
     int num;
+    int ret;
     QCowL2Meta meta;
 
     nb_sectors = bdrv_getlength(bs) >> 9;
     offset = 0;
     QLIST_INIT(&meta.dependent_requests);
+    meta.cluster_offset = 0;
 
     while (nb_sectors) {
         num = MIN(nb_sectors, INT_MAX >> 9);
-        cluster_offset = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num,
-            &meta);
+        ret = qcow2_alloc_cluster_offset(bs, offset, 0, num, &num, &meta);
 
-        if (cluster_offset == 0) {
+        if (ret < 0) {
             return -1;
         }
 
-        if (qcow2_alloc_cluster_link_l2(bs, cluster_offset, &meta) < 0) {
-            qcow2_free_any_clusters(bs, cluster_offset, meta.nb_clusters);
+        if (qcow2_alloc_cluster_link_l2(bs, &meta) < 0) {
+            qcow2_free_any_clusters(bs, meta.cluster_offset, meta.nb_clusters);
             return -1;
         }
 
@@ -821,10 +823,10 @@ static int preallocate(BlockDriverState *bs)
      * all of the allocated clusters (otherwise we get failing reads after
      * EOF). Extend the image to the last allocated sector.
      */
-    if (cluster_offset != 0) {
+    if (meta.cluster_offset != 0) {
         uint8_t buf[512];
         memset(buf, 0, 512);
-        bdrv_write(s->hd, (cluster_offset >> 9) + num - 1, buf, 1);
+        bdrv_write(s->hd, (meta.cluster_offset >> 9) + num - 1, buf, 1);
     }
 
     return 0;
diff --git a/block/qcow2.h b/block/qcow2.h
index 26ab5d9..d9ea6ab 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -135,6 +135,7 @@ struct QCowAIOCB;
 typedef struct QCowL2Meta
 {
     uint64_t offset;
+    uint64_t cluster_offset;
     int n_start;
     int nb_available;
     int nb_clusters;
@@ -199,8 +200,7 @@ uint64_t qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
                                          uint64_t offset,
                                          int compressed_size);
 
-int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset,
-    QCowL2Meta *m);
+int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m);
 
 /* qcow2-snapshot.c functions */
 int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
