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

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Message-ID: <CANZrA2YLtnv+PRBYzcLdFUg4uktx82G8dkoZCic8-ANK-3-7fA@mail.gmail.com>
Date:   Sun, 20 Jan 2019 06:03:17 +0100
From:   Florian La Roche <florian.laroche@...glemail.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will.deacon@....com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Crt Mori <cmo@...exis.com>, Joe Perches <joe@...ches.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: fix int_sqrt() for very large numbers

Hello all,

my comment said ffs(), but the code only uses fls() and that's what I meant.


Am So., 20. Jan. 2019 um 04:49 Uhr schrieb Linus Torvalds
<torvalds@...ux-foundation.org>:
> But yes, our current int_sqrt64() does seem buggy as-is, because it's
> *supposed* to work on u64's, even if I don't think we really have any
> users that care.

Right. No real bug, just not 100% correct code.

> And as Will mentioned, the regular int_sqrt() looks perfectly fine,
> and subtracting 1 from the __fls() return value would actually
> _introduce_ a bug.

I think no bug introduced as the code handling 0 and 1 is already done.

For __fls() and fls64() I am actually using the folloing code:
/*
 * fls - find last (most-significant) bit set
 * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
 */
static __always_inline unsigned int flsl(unsigned long x)
{
    return x ? sizeof(x) * 8 - __builtin_clzl(x) : 0;
}

Please note the "_builtin_clzl()" instead of the "_builtin_clz()".

The real bug is that we compute 1 to 64 for bit 0 to bit 63, whereas
the algorithm
expects 0 to 63 for the value of m.

best regards,

Florian La Roche