Re: [f2fs-dev] [PATCH v1 3/5] treewide: use get_random_u32() when possible - Jason A. Donenfeld via Linux-f2fs-devel

From: "Jason A. Donenfeld via Linux-f2fs-devel" <linux-f2fs-devel@lists.sourceforge.net>
To: Jason Gunthorpe <jgg@ziepe.ca>
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Subject: Re: [f2fs-dev] [PATCH v1 3/5] treewide: use get_random_u32() when possible
Date: Thu, 6 Oct 2022 07:05:48 -0600	[thread overview]
Message-ID: <CAHmME9r_vNRFFjUvqx8QkBddg_kQU=FMgpk9TqOVZdvX6zXHNg@mail.gmail.com> (raw)
In-Reply-To: <Yz7OdfKZeGkpZSKb@ziepe.ca>

On Thu, Oct 6, 2022 at 6:47 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Wed, Oct 05, 2022 at 11:48:42PM +0200, Jason A. Donenfeld wrote:
>
> > index 14392c942f49..499a425a3379 100644
> > --- a/drivers/infiniband/hw/cxgb4/cm.c
> > +++ b/drivers/infiniband/hw/cxgb4/cm.c
> > @@ -734,7 +734,7 @@ static int send_connect(struct c4iw_ep *ep)
> >                                  &ep->com.remote_addr;
> >       int ret;
> >       enum chip_type adapter_type = ep->com.dev->rdev.lldi.adapter_type;
> > -     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.

>
> > diff --git a/drivers/infiniband/ulp/ipoib/ipoib_cm.c b/drivers/infiniband/ulp/ipoib/ipoib_cm.c
> > index fd9d7f2c4d64..a605cf66b83e 100644
> > --- a/drivers/infiniband/ulp/ipoib/ipoib_cm.c
> > +++ b/drivers/infiniband/ulp/ipoib/ipoib_cm.c
> > @@ -465,7 +465,7 @@ static int ipoib_cm_req_handler(struct ib_cm_id *cm_id,
> >               goto err_qp;
> >       }
> >
> > -     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.

Jason

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