Re: [PATCH 05/32] locking/lockdep: Prepare valid_state() to handle plain masks

From: Linus Torvalds
Date: Tue Feb 12 2019 - 12:46:16 EST

On Tue, Feb 12, 2019 at 9:14 AM Frederic Weisbecker <frederic@xxxxxxxxxx> 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.