Re: [Qemu-devel] [PATCH v5 03/11] block: improve should_update_child

[Max Reitz]

On 14.01.19 17:13, Vladimir Sementsov-Ogievskiy wrote:
> 14.01.2019 17:32, Max Reitz wrote:
>> 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.)
> 
> So, even better is to use GQueue, strange that I didn't.

Because it has a tail pointer built in?  Hu, yeah, that's even better. :-)

Max

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