From 4d378bbd831bdd2f6e6adcd4ea5b77b6effaa627 Mon Sep 17 00:00:00 2001 From: Hanna Reitz Date: Wed, 16 Feb 2022 11:53:53 +0100 Subject: [PATCH] block: Make bdrv_refresh_limits() non-recursive MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit bdrv_refresh_limits() recurses down to the node's children. That does not seem necessary: When we refresh limits on some node, and then recurse down and were to change one of its children's BlockLimits, then that would mean we noticed the changed limits by pure chance. The fact that we refresh the parent's limits has nothing to do with it, so the reason for the change probably happened before this point in time, and we should have refreshed the limits then. Consequently, we should actually propagate block limits changes upwards, not downwards.  That is a separate and pre-existing issue, though, and so will not be addressed in this patch. The problem with recursing is that bdrv_refresh_limits() is not atomic. It begins with zeroing BDS.bl, and only then sets proper, valid limits. If we do not drain all nodes whose limits are refreshed, then concurrent I/O requests can encounter invalid request_alignment values and crash qemu. Therefore, a recursing bdrv_refresh_limits() requires the whole subtree to be drained, which is currently not ensured by most callers. A non-recursive bdrv_refresh_limits() only requires the node in question to not receive I/O requests, and this is done by most callers in some way or another: - bdrv_open_driver() deals with a new node with no parents yet - bdrv_set_file_or_backing_noperm() acts on a drained node - bdrv_reopen_commit() acts only on drained nodes - bdrv_append() should in theory require the node to be drained; in practice most callers just lock the AioContext, which should at least be enough to prevent concurrent I/O requests from accessing invalid limits So we can resolve the bug by making bdrv_refresh_limits() non-recursive. Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1879437 Signed-off-by: Hanna Reitz Reviewed-by: Eric Blake Message-Id: <20220216105355.30729-2-hreitz@redhat.com> Reviewed-by: Stefan Hajnoczi Signed-off-by: Kevin Wolf --- block/io.c | 4 ---- 1 file changed, 4 deletions(-) diff --git a/block/io.c b/block/io.c index 4b1d97c7da..efc011ce65 100644 --- a/block/io.c +++ b/block/io.c @@ -193,10 +193,6 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp) QLIST_FOREACH(c, &bs->children, next) { if (c->role & (BDRV_CHILD_DATA | BDRV_CHILD_FILTERED | BDRV_CHILD_COW)) { - bdrv_refresh_limits(c->bs, tran, errp); - if (*errp) { - return; - } bdrv_merge_limits(&bs->bl, &c->bs->bl); have_limits = true; }