netdev - Re: [PATCH 10/54] net: ethernet: replace bitmap_weight with bitmap

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Message-ID: <CAAH8bW8f5vGB33fwzehJHeMRi7-Z1vgO5TLCiTUXfZB1DJ_xCQ@mail.gmail.com>
Date:   Tue, 25 Jan 2022 15:10:00 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joe Perches <joe@...ches.com>, Dennis Zhou <dennis@...nel.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Nicholas Piggin <npiggin@...il.com>,
        Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Alexey Klimov <aklimov@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ariel Elior <aelior@...vell.com>,
        Manish Chopra <manishc@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 10/54] net: ethernet: replace bitmap_weight with
 bitmap_empty for qlogic

On Tue, Jan 25, 2022 at 2:15 PM David Laight <David.Laight@...lab.com> wrote:
>
> From: Yury Norov
> > Sent: 25 January 2022 21:10
> > On Mon, Jan 24, 2022 at 4:29 AM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:
> > >
> > > On Sun, Jan 23, 2022 at 10:38:41AM -0800, Yury Norov wrote:
> > > > qlogic/qed code calls bitmap_weight() to check if any bit of a given
> > > > bitmap is set. It's better to use bitmap_empty() in that case because
> > > > bitmap_empty() stops traversing the bitmap as soon as it finds first
> > > > set bit, while bitmap_weight() counts all bits unconditionally.
> > >
> > > > -             if (bitmap_weight((unsigned long *)&pmap[item], 64 * 8))
> > > > +             if (!bitmap_empty((unsigned long *)&pmap[item], 64 * 8))
> > >
> > > > -         (bitmap_weight((unsigned long *)&pmap[item],
> > > > +         (!bitmap_empty((unsigned long *)&pmap[item],
> > >
> > > Side note, these castings reminds me previous discussion and I'm wondering
> > > if you have this kind of potentially problematic places in your TODO as
> > > subject to fix.
> >
> > In the discussion you mentioned above, the u32* was cast to u64*,
> > which is wrong. The code
> > here is safe because in the worst case, it casts u64* to u32*. This
> > would be OK wrt
> >  -Werror=array-bounds.
> >
> > The function itself looks like doing this unsigned long <-> u64
> > conversions just for printing
> > purpose. I'm not a qlogic expert, so let's wait what people say?
>
> It'll be wrong on BE systems.

The bitmap_weigh() result will be correct. As you can see, the address
is 64-bit aligned anyways. The array boundary violation will never happen
as well.

DP_NOTICE() may be wrong, or may not. It depends on how important
the absolute position of the bit in the printed bitmap is. Nevertheless,
printk("%pb") is better and should be used.

This whole concern may be simply irrelevant if QED is not supported
on 32-bit BE machines. From what I can see, at least Infiniband requires
64BIT.

Thanks,
Yury

> You just can't cast the argument it has to be long[].
>
>         David
>
> -
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