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Re: [Qemu-devel] [PATCH v5 03/11] block: improve should_update_child
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v5 03/11] block: improve should_update_child |
Date: |
Mon, 14 Jan 2019 15:32:43 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 |
On 29.12.18 13:20, Vladimir Sementsov-Ogievskiy wrote:
> As it already said in the comment, we don't want to create loops in
> parent->child relations. So, when we try to append @to to @c, we should
> check that @c is not in @to children subtree, and we should check it
> recursively, not only the first level. The patch provides BFS-based
> search, to check the relations.
>
> This is needed for further fleecing-hook filter usage: we need to
> append it to source, when the hook is already a parent of target, and
> source may be in a backing chain of target (fleecing-scheme). So, on
> appending, the hook should not became a child (direct or through
> children subtree) of the target.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> block.c | 33 ++++++++++++++++++++++++++++-----
> 1 file changed, 28 insertions(+), 5 deletions(-)
Just two technical details:
> diff --git a/block.c b/block.c
> index 4f5ff2cc12..8f04f293da 100644
> --- a/block.c
> +++ b/block.c
> @@ -3536,7 +3536,7 @@ void bdrv_close_all(void)
>
> static bool should_update_child(BdrvChild *c, BlockDriverState *to)
> {
> - BdrvChild *to_c;
> + GList *queue = NULL, *pos;
GSList should be sufficient, I think.
>
> if (c->role->stay_at_node) {
> return false;
> @@ -3572,13 +3572,36 @@ static bool should_update_child(BdrvChild *c,
> BlockDriverState *to)
> * if A is a child of B, that means we cannot replace A by B there
> * because that would create a loop. Silently detaching A from B
> * is also not really an option. So overall just leaving A in
> - * place there is the most sensible choice. */
> - QLIST_FOREACH(to_c, &to->children, next) {
> - if (to_c == c) {
> - return false;
> + * place there is the most sensible choice.
> + *
> + * We would also create a loop in any cases where @c is only
> + * indirectly referenced by @to. Prevent this by returning false
> + * if @c is found (by breadth-first search) anywhere in the whole
> + * subtree of @to.
> + */
> +
> + pos = queue = g_list_append(queue, to);
> + while (pos) {
> + BlockDriverState *v = pos->data;
> + BdrvChild *c2;
> +
> + QLIST_FOREACH(c2, &v->children, next) {
> + if (c2 == c) {
> + g_list_free(queue);
> + return false;
> + }
> +
> + if (g_list_find(queue, c2->bs)) {
> + continue;
> + }
> +
> + queue = g_list_append(queue, c2->bs);
Appending to pos should be a bit quicker, I presume. (And you can
probably ignore the result, or just assert that the first argument was
returned.)
Max
> }
> +
> + pos = pos->next;
> }
>
> + g_list_free(queue);
> return true;
> }
>
>
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Max Reitz <=