Re: [PULL 01/19] util/hbitmap: strict hbitmap_reset

[John Snow]

On 10/11/19 7:18 PM, John Snow wrote:
> 
> 
> On 10/11/19 5:48 PM, Eric Blake wrote:
>> On 10/11/19 4:25 PM, John Snow wrote:
>>> From: Vladimir Sementsov-Ogievskiy <address@hidden>
>>>
>>> hbitmap_reset has an unobvious property: it rounds requested region up.
>>> It may provoke bugs, like in recently fixed write-blocking mode of
>>> mirror: user calls reset on unaligned region, not keeping in mind that
>>> there are possible unrelated dirty bytes, covered by rounded-up region
>>> and information of this unrelated "dirtiness" will be lost.
>>>
>>> Make hbitmap_reset strict: assert that arguments are aligned, allowing
>>> only one exception when @start + @count == hb->orig_size. It's needed
>>> to comfort users of hbitmap_next_dirty_area, which cares about
>>> hb->orig_size.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>>> Reviewed-by: Max Reitz <address@hidden>
>>> Message-Id: <address@hidden>
>>> [Maintainer edit: Max's suggestions from on-list. --js]
>>> Signed-off-by: John Snow <address@hidden>
>>> ---
>>>   include/qemu/hbitmap.h | 5 +++++
>>>   tests/test-hbitmap.c   | 2 +-
>>>   util/hbitmap.c         | 4 ++++
>>>   3 files changed, 10 insertions(+), 1 deletion(-)
>>>
>>
>>> +++ b/util/hbitmap.c
>>> @@ -476,6 +476,10 @@ void hbitmap_reset(HBitmap *hb, uint64_t start,
>>> uint64_t count)
>>>       /* Compute range in the last layer.  */
>>>       uint64_t first;
>>>       uint64_t last = start + count - 1;
>>> +    uint64_t gran = 1ULL << hb->granularity;
>>> +
>>> +    assert(!(start & (gran - 1)));
>>> +    assert(!(count & (gran - 1)) || (start + count == hb->orig_size));
>>
>> I know I'm replying a bit late (since this is now a pull request), but
>> would it be worth using the dedicated macro:
>>
>> assert(QEMU_IS_ALIGNED(start, gran));
>> assert(QEMU_IS_ALIGNED(count, gran) || start + count == hb->orig_size);
>>
>> instead of open-coding it?  (I would also drop the extra () around the
>> right half of ||). If we want it, that would now be a followup patch.

I've noticed that seasoned C programmers hate extra parentheses a lot.
I've noticed that I cannot remember operator precedence enough to ever
feel like this is actually an improvement.

Something about a nice weighted tree of ((expr1) || (expr2)) feels
soothing to my weary eyes. So, if it's not terribly important, I'd
prefer to leave it as-is.

(You may feel free to counter-educate me as desired.)

>>
> 
> If the PR doesn't make it for some reason, I can amend a cleanup patch
> for the next PR.
> 

by the way: GOOD NEWS! ...

--js