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Re: [Qemu-block] [PATCH v2 2/6] block/dirty-bitmaps: rename frozen predi


From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v2 2/6] block/dirty-bitmaps: rename frozen predicate helper
Date: Tue, 19 Feb 2019 10:17:28 +0000

19.02.2019 1:32, John Snow wrote:
> 
> 
> On 2/18/19 8:57 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 14.02.2019 2:23, John Snow wrote:
>>> "Frozen" was a good description a long time ago, but it isn't adequate now.
>>> Rename the frozen predicate to has_successor to make the semantics of the
>>> predicate more clear to outside callers.
>>>
>>> In the process, remove some calls to frozen() that no longer semantically
>>> make sense. For enabled and disabled in particular, it's actually okay for
>>> the internals to do this but only forbidden for users to invoke them, and
>>
>> I'm a bit lost in this paragraph.. to this - what?, to invoke them - whom?
>> I think, it would be simpler for me to read patch itself :)
>>
> 
> Touched this up. I meant enable and disable, not enabled and disabled.
> 
>>> all of the QMP entry uses already check against qmp_locked.
>>>
>>> Several other assertions really want to check that the bitmap isn't in-use
>>> by another operation -- use the qmp_locked function for this instead, which
>>> presently also checks for has_successor.
>>
>> hm, you mean user_locked, not qmp_locked.
>>
> 
> Yes.
> 
> [...]
> 
>>>    /**
>>>     * Create a successor bitmap destined to replace this bitmap after an 
>>> operation.
>>> - * Requires that the bitmap is not frozen and has no successor.
>>> + * Requires that the bitmap is not locked and has no successor.
>>
>> I think, user_locked, to not interfere with bitmaps mutex. And you use 
>> user_locked in
>> other comments in this patch.
>>
> 
> You're right. It gets changed again later, but I didn't make this easy
> to read.
> 
>>>     * Called with BQL taken.
>>>     */
>>>    int bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
>>> @@ -244,12 +244,16 @@ int 
>>> bdrv_dirty_bitmap_create_successor(BlockDriverState *bs,
>>>        uint64_t granularity;
>>>        BdrvDirtyBitmap *child;
>>>    
>>> -    if (bdrv_dirty_bitmap_frozen(bitmap)) {
>>> -        error_setg(errp, "Cannot create a successor for a bitmap that is "
>>> -                   "currently frozen");
>>> +    if (bdrv_dirty_bitmap_user_locked(bitmap)) {
>>> +        error_setg(errp, "Cannot create a successor for a bitmap that is 
>>> in-use "
>>> +                   "by an operation");
>>> +        return -1;
>>> +    }
>>> +    if (bdrv_dirty_bitmap_has_successor(bitmap)) {
>>> +        error_setg(errp, "Cannot create a successor for a bitmap that 
>>> already "
>>> +                   "has one");
>>
>>
>> Amm, dead code? _user_locked() implies no successor, so we instead can keep 
>> an assertion..
>>
> 
> It gets changed later in the series, but I didn't do a great job of
> explaining that in advance. I'll amend the commit message to explain
> what I'm trying to do.
> 
> I tried to hint at this with: "which presently also checks for
> has_successor" as an admission that it was redundant, but I need to call
> it out in stronger language.
> 

Hmm, isn't it better to keep an assertion, than add dead code, to be fixed in 
later commits?

-- 
Best regards,
Vladimir

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