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

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Message-ID: <871r8tpnws.fsf@toke.dk>
Date:   Tue, 22 Jun 2021 23:52:51 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>, netdev@...r.kernel.org
Cc:     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

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? :)

-Toke