netdev - Re: [PATCH net-next v3 2/5] bitops: add non-atomic bitops for pointers

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Date:   Wed, 23 Jun 2021 03:40:23 +0530
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org
Subject: Re: [PATCH net-next v3 2/5] bitops: add non-atomic bitops for
 pointers

On Wed, Jun 23, 2021 at 03:22:51AM IST, Toke Høiland-Jørgensen wrote:
> Kumar Kartikeya Dwivedi <memxor@...il.com> writes:
>
> > cpumap needs to set, clear, and test the lowest bit in skb pointer in
> > various places. To make these checks less noisy, add pointer friendly
> > bitop macros that also do some typechecking to sanitize the argument.
> >
> > These wrap the non-atomic bitops __set_bit, __clear_bit, and test_bit
> > but for pointer arguments. Pointer's address has to be passed in and it
> > is treated as an unsigned long *, since width and representation of
> > pointer and unsigned long match on targets Linux supports. They are
> > prefixed with double underscore to indicate lack of atomicity.
> >
> > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> > ---
> >  include/linux/bitops.h    | 19 +++++++++++++++++++
> >  include/linux/typecheck.h | 10 ++++++++++
> >  2 files changed, 29 insertions(+)
> >
> > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> > index 26bf15e6cd35..a9e336b9fa4d 100644
> > --- a/include/linux/bitops.h
> > +++ b/include/linux/bitops.h
> > @@ -4,6 +4,7 @@
> >
> >  #include <asm/types.h>
> >  #include <linux/bits.h>
> > +#include <linux/typecheck.h>
> >
> >  #include <uapi/linux/kernel.h>
> >
> > @@ -253,6 +254,24 @@ static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
> >  		__clear_bit(nr, addr);
> >  }
> >
> > +#define __ptr_set_bit(nr, addr)                         \
> > +	({                                              \
> > +		typecheck_pointer(*(addr));             \
> > +		__set_bit(nr, (unsigned long *)(addr)); \
> > +	})
> > +
> > +#define __ptr_clear_bit(nr, addr)                         \
> > +	({                                                \
> > +		typecheck_pointer(*(addr));               \
> > +		__clear_bit(nr, (unsigned long *)(addr)); \
> > +	})
> > +
> > +#define __ptr_test_bit(nr, addr)                       \
> > +	({                                             \
> > +		typecheck_pointer(*(addr));            \
> > +		test_bit(nr, (unsigned long *)(addr)); \
> > +	})
> > +
>
> Before these were functions that returned the modified values, now they
> are macros that modify in-place. Why the change? :)
>

Given that we're exporting this to all kernel users now, it felt more
appropriate to follow the existing convention/argument order for the
functions/ops they are wrapping.

I really have no preference here though...

> -Toke
>

--
Kartikeya