linux-kernel - Re: [PATCH v1 3/5] treewide: use get_random

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Message-ID: <Yz7WHyD+teLOh2ho@smile.fi.intel.com>
Date:   Thu, 6 Oct 2022 16:20:31 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v1 3/5] treewide: use get_random_u32() when possible

On Thu, Oct 06, 2022 at 07:05:48AM -0600, Jason A. Donenfeld wrote:
> On Thu, Oct 6, 2022 at 6:47 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > On Wed, Oct 05, 2022 at 11:48:42PM +0200, Jason A. Donenfeld wrote:

...

> > > -     u32 isn = (prandom_u32() & ~7UL) - 1;
> > > +     u32 isn = (get_random_u32() & ~7UL) - 1;
> >
> > Maybe this wants to be written as
> >
> > (prandom_max(U32_MAX >> 7) << 7) | 7

> > ?
> 
> Holy smokes. Yea I guess maybe? It doesn't exactly gain anything or
> make the code clearer though, and is a little bit more magical than
> I'd like on a first pass.

Shouldn't the two first 7s to be 3s?

...

> > > -     psn = prandom_u32() & 0xffffff;
> > > +     psn = get_random_u32() & 0xffffff;
> >
> >  prandom_max(0xffffff + 1)
> 
> That'd work, but again it's not more clear. Authors here are going for
> a 24-bit number, and masking seems like a clear way to express that.

We have some 24-bit APIs (and 48-bit) already in kernel, why not to have
get_random_u24() ?


-- 
With Best Regards,
Andy Shevchenko