Re: [PATCH 1/2] IB/hfi1: Fix a parameter of find_first_bit.

[Leon Romanovsky <leon@kernel.org>]

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On Fri, Sep 02, 2016 at 10:39:11AM -0400, Doug Ledford wrote:
> On 8/28/2016 2:06 AM, Leon Romanovsky wrote:
> > On Fri, Aug 26, 2016 at 03:34:48PM -0400, Doug Ledford wrote:
> >> On 8/26/2016 3:29 PM, Leon Romanovsky wrote:
> >>> On Fri, Aug 26, 2016 at 02:01:55PM -0400, Doug Ledford wrote:
> >>>> On 8/26/2016 9:35 AM, Doug Ledford wrote:
> >>>>> On 8/26/2016 12:49 AM, Christophe JAILLET wrote:
> >>>>>> The 2nd parameter of 'find_first_bit' is the number of bits to search.
> >>>>>> In this case, we are passing 'sizeof(unsigned long)' which is likely to
> >>>>>> be 4 or 8.
> >>>>>
> >>>>> If the size can be 4 or 8, then using 64 universally is not correct.
> >>>>> Why not use sizeof() * 8 (or << 3)?
> >>>>
> >>>> Better yet, why not put this patch in the kernel first:
> >>>>
> >>>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> >>>> index d96a6118d26a..a8838c87668e 100644
> >>>> --- a/include/linux/kernel.h
> >>>> +++ b/include/linux/kernel.h
> >>>> @@ -52,6 +52,8 @@
> >>>>
> >>>>  #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) +
> >>>> __must_be_array(arr))
> >>>>
> >>>> +#define bitsizeof(x)   (sizeof((x)) << 3)
> >>>> +
> >>>>  #define u64_to_user_ptr(x) (           \
> >>>>  {                                      \
> >>>>         typecheck(u64, x);              \
> >>>>
> >>>> then start going around replacing all these hard coded numbers with the
> >>>> use of bitsizeof().  It can be applied not just to the find_first*bit()
> >>>> routines, but to a bunch of other routines too.  Just look at
> >>>> include/linux/bitmap.h and any that have nbits as an argument are
> >>>> candidates.
> >>>
> >>> There is BITS_PER_LONG define for that. There is actual use of it in mlx5 for
> >>> the similar code pieces.
> >>
> >> BITS_PER_LONG only works if your bitfield is a single long.  It doesn't
> >> work for other bitfields.  What I posted above will work for anything.
> >
> > Yes, the question to ask if it is really needed.
>
> A quick review of the usage of find_first_bit in the kernel shows that
> the majority of uses that use large, complex bit arrays (things larger
> than a single long, where bitsizeof(complex_object) might be helpful),
> also tend to have limits to their bitmap sizes that do not directly
> equal the size of the bitmap.  For example, the bitmap for an md raid
> array is allocated as a chunk of memory, where the chunk of memory is
> larger than the bitmap size as a general rule, so
> bitsizeof(chunk_of_memory) would run off the end of the real bitmap.
> Likewise for a number of other complex bitmaps (block layer tag in use
> bitmap for instance).
>
> So, is it useful?  I think so.  But it's not needed for original patch
> in this thread.

Honestly, I wasn't convinced at all, but you are right,
it is theoretical discussion.

Thanks

>
>
> --
> Doug Ledford <dledford@redhat.com>
>     GPG Key ID: 0E572FDD
>




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