From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:59890) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TvVGQ-0006oK-FP for qemu-devel@nongnu.org; Wed, 16 Jan 2013 10:50:57 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TvVGN-0005ce-K5 for qemu-devel@nongnu.org; Wed, 16 Jan 2013 10:50:54 -0500 Received: from nodalink.pck.nerim.net ([62.212.105.220]:42928 helo=paradis.irqsave.net) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TvVGN-0005cX-AA for qemu-devel@nongnu.org; Wed, 16 Jan 2013 10:50:51 -0500 From: =?UTF-8?q?Beno=C3=AEt=20Canet?= Date: Wed, 16 Jan 2013 16:48:07 +0100 Message-Id: <1358351321-4891-29-git-send-email-benoit@irqsave.net> In-Reply-To: <1358351321-4891-1-git-send-email-benoit@irqsave.net> References: <1358351321-4891-1-git-send-email-benoit@irqsave.net> Subject: [Qemu-devel] [RFC V5 28/62] qcow2: Do not overwrite existing entries with QCOW_OFLAG_COPIED. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: kwolf@redhat.com, pbonzini@redhat.com, =?UTF-8?q?Beno=C3=AEt=20Canet?= , stefanha@redhat.com 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 --- 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