On Mon, Nov 09, 2020 at 10:15:29PM +0530, Syed Nayyar Waris wrote:
> On Mon, Nov 09, 2020 at 03:41:53PM +0100, Arnd Bergmann wrote:
> > On Mon, Nov 9, 2020 at 2:41 PM William Breathitt Gray
> > <vilhelm.gray@xxxxxxxxx> wrote:
> > > On Mon, Nov 09, 2020 at 06:04:11PM +0530, Syed Nayyar Waris wrote:
> > >
> > > One of my concerns is that we're incurring the latency two additional
> > > conditional checks just to suppress a compiler warning about a case that
> > > wouldn't occur in the actual use of bitmap_set_value(). I'm hoping
> > > there's a way for us to suppress these warnings without adding onto the
> > > latency of this function; given that bitmap_set_value() is intended to
> > > be used in loops, conditionals here could significantly increase latency
> > > in drivers.
> >
> > At least for this caller, the size check would be a compile-time
> > constant that can be eliminated.
> >
> > > I wonder if array_index_nospec() might have the side effect of
> > > suppressing these warnings for us. For example, would this work:
> > >
> > > static inline void bitmap_set_value(unsigned long *map,
> > > unsigned long value,
> > > unsigned long start, unsigned long nbits)
> > > {
> > > const unsigned long offset = start % BITS_PER_LONG;
> > > const unsigned long ceiling = round_up(start + 1, BITS_PER_LONG);
> > > const unsigned long space = ceiling - start;
> > > size_t index = BIT_WORD(start);
> > >
> > > value &= GENMASK(nbits - 1, 0);
> > >
> > > if (space >= nbits) {
> > > index = array_index_nospec(index, index + 1);
> > >
> > > map[index] &= ~(GENMASK(nbits - 1, 0) << offset);
> > > map[index] |= value << offset;
> > > } else {
> > > index = array_index_nospec(index, index + 2);
> > >
> > > map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
> > > map[index + 0] |= value << offset;
> > > map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> > > map[index + 1] |= value >> space;
> > > }
> > > }
> > >
> > > Or is this going to produce the same warning because we're not using an
> > > explicit check against the map array size?
> >
> > https://godbolt.org/z/fxnsG9
> >
> > It still warns about the 'map[index + 1]' access: from all I can tell,
> > gcc mainly complains because it cannot rule out that 'space < nbits',
> > and then it knows the size of 'DECLARE_BITMAP(old, 64)' and finds
> > that if 'index + 0' is correct, then 'index + 1' overflows that array.
> >
> > Arnd
>
> Hi Arnd,
>
> As suggested by William, sharing another solution to suppress the
> compiler warning. Please let me know your views on the below fix. Thanks.
>
> If its alright, I shall submit a (new) v13 patchset soon. Let me know.
>
> @@ -1,5 +1,5 @@
> static inline void bitmap_set_value(unsigned long *map,
> - unsigned long value,
> + unsigned long value, const size_t length,
> unsigned long start, unsigned long nbits)
> {
> const size_t index = BIT_WORD(start);
> @@ -15,6 +15,10 @@ static inline void bitmap_set_value(unsigned long *map,
> } else {
> map[index + 0] &= ~BITMAP_FIRST_WORD_MASK(start);
> map[index + 0] |= value << offset;
> +
> + if (index + 1 >= length)
> + __builtin_unreachable();
> +
> map[index + 1] &= ~BITMAP_LAST_WORD_MASK(start + nbits);
> map[index + 1] |= value >> space;
> }
Hi Syed,
Let's rename 'length' to 'nbits' as Arnd suggested, and rename 'nbits'
to value_width.
William Breathitt Gray
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