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

Am 14.10.2019 um 20:10 hat John Snow geschrieben: > > > 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 <vsementsov(a)virtuozzo.com&gt; >>>> >>>> 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 <vsementsov(a)virtuozzo.com&gt; >>>> Reviewed-by: Max Reitz <mreitz(a)redhat.com&gt; >>>> Message-Id: <20190806152611.280389-1-vsementsov(a)virtuozzo.com&gt; >>>> [Maintainer edit: Max's suggestions from on-list. --js] >>>> Signed-off-by: John Snow <jsnow(a)redhat.com&gt; >>>> --- >>>>   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. I don't mind the parentheses, but I do prefer QEMU_IS_ALIGNED() to the open-coded version. Would that be a viable compromise?