linux-kernel - Re: fix int_sqrt() for very large numbers

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Message-ID: <CAHk-=wjruxUdKWjn-PvZMOPn3pGO5yTcbc_J4hc4TrS0P15vaw@mail.gmail.com>
Date:   Mon, 21 Jan 2019 13:20:33 +1300
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Florian La Roche <florian.laroche@...glemail.com>
Cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Crt Mori <cmo@...exis.com>, Joe Perches <joe@...ches.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: fix int_sqrt() for very large numbers

On Sun, Jan 20, 2019 at 4:15 AM Florian La Roche
<florian.laroche@...glemail.com> wrote:
>
> @@ -52,7 +52,7 @@ u32 int_sqrt64(u64 x)
>         if (x <= ULONG_MAX)
>                 return int_sqrt((unsigned long) x);
>
> -       m = 1ULL << (fls64(x) & ~1ULL);
> +       m = 1ULL << ((fls64(x) - 1) & ~1ULL);

I've applied this part of the patch as commit fbfaf851902c ("fix
int_sqrt64() for very large numbers") with slightly edited commit
log.

I still think there are some oddities in here in the types. I
mentioned the caller that unnecessarily does the int_sqrt64() twice,
even though the outer one doesn't actually take a 64-bit value.

But in the very line above, there's another type oddity: the "& ~1ULL"
is entirely the wrong type. The shift *count* shouldn't be an unsigned
long long, so that type doesn't make much sense. It should be just a
~1, or even just "62".

But I didn't actually start micro-editing the patch, and just did that
one-liner off-by-one fix.

             Linus