From d4d698f020e50333d6eae48ce323752613b5c3ea Mon Sep 17 00:00:00 2001 From: Gleb Natapov Date: Thu, 7 May 2009 11:31:44 +0300 Subject: [PATCH] Fix cluster freeing in qcow2 Need to drop QCOW_OFLAG_COPIED from a cluster pointer before freeing it. Add an explanation how thing meant to work. Signed-off-by: Gleb Natapov Signed-off-by: Anthony Liguori --- block-qcow2.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/block-qcow2.c b/block-qcow2.c index 1f3312590c..9a49777ed1 100644 --- a/block-qcow2.c +++ b/block-qcow2.c @@ -992,6 +992,12 @@ static int alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset, goto err; for (i = 0; i < m->nb_clusters; i++) { + /* 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 + * cluster the second one has to do RMW (which is done above by + * copy_sectors()), update l2 table with its cluster pointer and free + * old cluster. This is what this loop does */ if(l2_table[l2_index + i] != 0) old_cluster[j++] = l2_table[l2_index + i]; @@ -1005,7 +1011,8 @@ static int alloc_cluster_link_l2(BlockDriverState *bs, uint64_t cluster_offset, goto err; for (i = 0; i < j; i++) - free_any_clusters(bs, be64_to_cpu(old_cluster[i]), 1); + free_any_clusters(bs, be64_to_cpu(old_cluster[i]) & ~QCOW_OFLAG_COPIED, + 1); ret = 0; err: