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Date: Mon, 8 Jun 2020 16:00:06 +0300
From: Andy Shevchenko <andy.shevchenko@gmail.com>
Subject: Re: [PATCH RESEND2] lib: fix bitmap_parse() on 64-bit big endian
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To: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-s390@vger.kernel.org, Stable <stable@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Yury Norov <yury.norov@gmail.com>, Amritha Nambiar <amritha.nambiar@intel.com>, Arnaldo Carvalho de Melo <acme@redhat.com>, Chris Wilson <chris@chris-wilson.co.uk>, Kees Cook <keescook@chromium.org>, Matthew Wilcox <willy@infradead.org>, Miklos Szeredi <mszeredi@redhat.com>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Steffen Klassert <steffen.klassert@secunet.com>, "Tobin C . Harding" <tobin@kernel.org>, Vineet Gupta <vineet.gupta1@synopsys.com>, Will Deacon <will.deacon@arm.com>, Willem de Bruijn <willemb@google.com>

On Mon, Jun 08, 2020 at 02:44:34PM +0200, Alexander Gordeev wrote:
> On Mon, Jun 08, 2020 at 03:03:05PM +0300, Andy Shevchenko wrote:
> > On Mon, Jun 8, 2020 at 1:26 PM Alexander Gordeev <agordeev@linux.ibm.com> wrote:
> > >
> > > Commit 2d6261583be0 ("lib: rework bitmap_parse()") does not
> > > take into account order of halfwords on 64-bit big endian
> > > architectures. As result (at least) Receive Packet Steering,
> > > IRQ affinity masks and runtime kernel test "test_bitmap" get
> > > broken on s390.
> > 
> > ...
> > 
> > > +#if defined(__BIG_ENDIAN) && defined(CONFIG_64BIT)
> > 
> > I think it's better to re-use existing patterns.
> > 
> > ipc/sem.c:1682:#if defined(CONFIG_64BIT) && defined(__BIG_ENDIAN)
> > 
> > > +static void save_x32_chunk(unsigned long *maskp, u32 chunk, int chunk_idx)
> > > +{
> > > +       maskp += (chunk_idx / 2);
> > > +       ((u32 *)maskp)[(chunk_idx & 1) ^ 1] = chunk;
> > > +}
> > > +#else
> > > +static void save_x32_chunk(unsigned long *maskp, u32 chunk, int chunk_idx)
> > > +{
> > > +       ((u32 *)maskp)[chunk_idx] = chunk;
> > > +}
> > > +#endif
> > 
> > See below.
> > 
> > ...
> > 
> > > -               end = bitmap_get_x32_reverse(start, end, bitmap++);
> > > +               end = bitmap_get_x32_reverse(start, end, &chunk);
> > >                 if (IS_ERR(end))
> > >                         return PTR_ERR(end);
> > > +
> > > +               save_x32_chunk(maskp, chunk, chunk_idx++);
> > 
> > Can't we simple do
> > 
> >         int chunk_index = 0;
> >         ...
> >         do {
> > #if defined(CONFIG_64BIT) && defined(__BIG_ENDIAN)
> >                end = bitmap_get_x32_reverse(start, end,
> > bitmap[chunk_index ^ 1]);
> > #else
> >                end = bitmap_get_x32_reverse(start, end, bitmap[chunk_index]);
> > #endif
> >         ...
> >         } while (++chunk_index);
> > 
> > ?
> 
> Well, unless we ignore coding style 21) Conditional Compilation
> we could. Do you still insist it would be better?

I think it's okay to do here
 - it's not a big function
 - it has no stub versions (you always do something)
 - the result pretty much readable (5 lines any editor can keep on screen)
 - and it's not ignoring, see "Wherever possible...", compare readability of
   two versions, for yours reader needs to go somewhere to read, calculate and
   return, when everything already being forgotten
 - last but not least, I bet it makes code shorter (at least in C)

-- 
With Best Regards,
Andy Shevchenko