From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:50460) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gKNbh-0006t3-7H for qemu-devel@nongnu.org; Wed, 07 Nov 2018 08:10:54 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gKNbb-0002uu-9I for qemu-devel@nongnu.org; Wed, 07 Nov 2018 08:10:49 -0500 Received: from mail-qk1-x743.google.com ([2607:f8b0:4864:20::743]:40382) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1gKNba-0002Hr-TN for qemu-devel@nongnu.org; Wed, 07 Nov 2018 08:10:47 -0500 Received: by mail-qk1-x743.google.com with SMTP id y16so19417748qki.7 for ; Wed, 07 Nov 2018 05:10:29 -0800 (PST) From: Daniel Henrique Barboza Date: Wed, 7 Nov 2018 11:10:00 -0200 Message-Id: <20181107131000.27744-4-danielhb413@gmail.com> In-Reply-To: <20181107131000.27744-1-danielhb413@gmail.com> References: <20181107131000.27744-1-danielhb413@gmail.com> Subject: [Qemu-devel] [PATCH for-3.2 v3 3/3] qcow2-snapshot: remove redundant find_snapshot_by_id_and_name call List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: dgilbert@redhat.com, kwolf@redhat.com, mreitz@redhat.com, armbru@redhat.com, Daniel Henrique Barboza In qcow2_snapshot_create there is the following code block: /* Generate an ID */ find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str)); /* Check that the ID is unique */ if (find_snapshot_by_id_and_name(bs, sn_info->id_str, NULL) >= 0) { return -EEXIST; } find_new_snapshot_id cycles through all snapshots, getting the id_str as an unsigned long int, calculating the max id_max value of all the existing id_strs and writing in the id_str pointer id_max + 1: for(i = 0; i < s->nb_snapshots; i++) { sn = s->snapshots + i; id = strtoul(sn->id_str, NULL, 10); if (id > id_max) id_max = id; } snprintf(id_str, id_str_size, "%lu", id_max + 1); Here, sn_info->id_str will have the unique value id_max + 1. Right after that, find_snapshot_by_id_and_name is called with id = sn_info->id_str and name = NULL. This will cause the function to execute the following: } else if (id) { for (i = 0; i < s->nb_snapshots; i++) { if (!strcmp(s->snapshots[i].id_str, id)) { return i; } } } In short, we're searching the existing snapshots to see if sn_info->id_str matches any existing id, right after we set in the previous line a sn_info->id_str value that is already unique. The first code block goes way back to commit 585f8587ad, a 2006 commit from Fabrice Bellard that simply says "new qcow2 disk image format". No more info is provided about this logic in any subsequent commits that moved this code block around. I can't say about the original design, but the current logic is redundant. bdrv_snapshot_create is called in aio_context lock, forbidding any concurrent call to accidentally create a new snapshot between the find_new_snapshot_id and find_snapshot_by_id_and_name calls. What we're ending up doing is to cycle through the snapshots two times for no viable reason. This patch eliminates the redundancy by removing the 'id is unique' check that calls find_snapshot_by_id_and_name. Signed-off-by: Daniel Henrique Barboza --- block/qcow2-snapshot.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c index bb6a5b7516..20e8472191 100644 --- a/block/qcow2-snapshot.c +++ b/block/qcow2-snapshot.c @@ -358,11 +358,6 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info) /* Generate an ID */ find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str)); - /* Check that the ID is unique */ - if (find_snapshot_by_id_and_name(bs, sn_info->id_str, NULL) >= 0) { - return -EEXIST; - } - /* Populate sn with passed data */ sn->id_str = g_strdup(sn_info->id_str); sn->name = g_strdup(sn_info->name); -- 2.17.2