Re: [libvirt] [PATCH v2 2/6] block/dirty-bitmaps: rename frozen predicate helper
On 2/19/19 5:17 AM, Vladimir Sementsov-Ogievskiy wrote:
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?
Eh. I wrote code that looked semantically correct without worrying about
what the calls actually do:
- We want to make sure this bitmap is not in use (user_locked,
qmp_locked, or busy -- however you want to spell it), and
- We want to make sure this bitmap doesn't have a successor.
Now, you and I happen to know that these two conditions aren't actually
independent, but that's not necessarily obvious from this one function
to a new reader. Adding the second check gives some assurance to the reader.
In my mind, the concept of having a successor is now distinct from that
of being busy, and create_successor actually wants to check both things:
(1) That is able to create a successor, because it doesn't have one, and
(2) That it is not modifying a bitmap in use by some operation.
But, you're right, there's no way to have a bitmap with a successor that
isn't busy, so an assertion will suffice for the instructional purpose.
I'll change it.
--js