On Tue, Feb 12, 2019 at 9:14 AM Frederic Weisbecker <frede...@kernel.org> wrote:
>
> +
> + while (vectors) {
> + long fs = __ffs64(vectors) + 1;
> +
> + vectors >>= fs;
This is wrong.
If "vectors" only has the high hit set, you end up with "fs" having
the value "64".
And then "vectors >>= fs" is undefined and won't actually do anything
at all on x86.
In general, avoid "ffs()", and the stupid pattern of "__ffs(x)+1".
Bit numbering starts at 0. "ffs()" is wrong. And you never *ever* just
add one to a bit number in order to shift by one more bit, exactly
because of overflow issues.
So it may look inefficient, but the correct thing to do is
long bit = __ffs64(vectors);
vectors >>= fs;
vectors >>= 1;
because that actually works.
Linus
