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

[Vladimir Sementsov-Ogievskiy]

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.

> 
> Max
> 
>>           }
>> +
>> +        pos = pos->next;
>>       }
>>   
>> +    g_list_free(queue);
>>       return true;
>>   }
>>   
>>
> 
> 


-- 
Best regards,
Vladimir