Re: [PATCH v3 2/7] bitops: always define asm-generic non-atomic bitops

[Marco Elver]

On Fri, 17 Jun 2022 at 19:19, Alexander Lobakin
<alexandr.lobakin@xxxxxxxxx> wrote:
>
> Move generic non-atomic bitops from the asm-generic header which
> gets included only when there are no architecture-specific
> alternatives, to a separate independent file to make them always
> available.
> Almost no actual code changes, only one comment added to
> generic_test_bit() saying that it's an atomic operation itself
> and thus `volatile` must always stay there with no cast-aways.
>
> Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx> # comment
> Suggested-by: Marco Elver <elver@xxxxxxxxxx> # reference to kernel-doc
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  .../asm-generic/bitops/generic-non-atomic.h   | 130 ++++++++++++++++++
>  include/asm-generic/bitops/non-atomic.h       | 110 ++-------------
>  2 files changed, 138 insertions(+), 102 deletions(-)
>  create mode 100644 include/asm-generic/bitops/generic-non-atomic.h
>
> diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h
> new file mode 100644
> index 000000000000..7226488810e5
> --- /dev/null
> +++ b/include/asm-generic/bitops/generic-non-atomic.h
> @@ -0,0 +1,130 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +#ifndef __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H
> +#define __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H
> +
> +#include <linux/bits.h>
> +
> +#ifndef _LINUX_BITOPS_H
> +#error only <linux/bitops.h> can be included directly
> +#endif
> +
> +/*
> + * Generic definitions for bit operations, should not be used in regular code
> + * directly.
> + */
> +
> +/**
> + * generic___set_bit - Set a bit in memory
> + * @nr: the bit to set
> + * @addr: the address to start counting from
> + *
> + * Unlike set_bit(), this function is non-atomic and may be reordered.
> + * If it's called on the same region of memory simultaneously, the effect
> + * may be that only one operation succeeds.
> + */
> +static __always_inline void
> +generic___set_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +
> +       *p  |= mask;
> +}
> +
> +static __always_inline void
> +generic___clear_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +
> +       *p &= ~mask;
> +}
> +
> +/**
> + * generic___change_bit - Toggle a bit in memory
> + * @nr: the bit to change
> + * @addr: the address to start counting from
> + *
> + * Unlike change_bit(), this function is non-atomic and may be reordered.
> + * If it's called on the same region of memory simultaneously, the effect
> + * may be that only one operation succeeds.
> + */
> +static __always_inline
> +void generic___change_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +
> +       *p ^= mask;
> +}
> +
> +/**
> + * generic___test_and_set_bit - Set a bit and return its old value
> + * @nr: Bit to set
> + * @addr: Address to count from
> + *
> + * This operation is non-atomic and can be reordered.
> + * If two examples of this operation race, one can appear to succeed
> + * but actually fail.  You must protect multiple accesses with a lock.
> + */
> +static __always_inline int
> +generic___test_and_set_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +       unsigned long old = *p;
> +
> +       *p = old | mask;
> +       return (old & mask) != 0;
> +}
> +
> +/**
> + * generic___test_and_clear_bit - Clear a bit and return its old value
> + * @nr: Bit to clear
> + * @addr: Address to count from
> + *
> + * This operation is non-atomic and can be reordered.
> + * If two examples of this operation race, one can appear to succeed
> + * but actually fail.  You must protect multiple accesses with a lock.
> + */
> +static __always_inline int
> +generic___test_and_clear_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +       unsigned long old = *p;
> +
> +       *p = old & ~mask;
> +       return (old & mask) != 0;
> +}
> +
> +/* WARNING: non atomic and it can be reordered! */
> +static __always_inline int
> +generic___test_and_change_bit(unsigned int nr, volatile unsigned long *addr)
> +{
> +       unsigned long mask = BIT_MASK(nr);
> +       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> +       unsigned long old = *p;
> +
> +       *p = old ^ mask;
> +       return (old & mask) != 0;
> +}
> +
> +/**
> + * generic_test_bit - Determine whether a bit is set
> + * @nr: bit number to test
> + * @addr: Address to start counting from
> + */
> +static __always_inline int
> +generic_test_bit(unsigned int nr, const volatile unsigned long *addr)
> +{
> +       /*
> +        * Unlike the bitops with the '__' prefix above, this one *is* atomic,
> +        * so `volatile` must always stay here with no cast-aways. See
> +        * `Documentation/atomic_bitops.txt` for the details.
> +        */
> +       return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
> +}
> +
> +#endif /* __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H */
> diff --git a/include/asm-generic/bitops/non-atomic.h b/include/asm-generic/bitops/non-atomic.h
> index 078cc68be2f1..23d3abc1e10d 100644
> --- a/include/asm-generic/bitops/non-atomic.h
> +++ b/include/asm-generic/bitops/non-atomic.h
> @@ -2,121 +2,27 @@
>  #ifndef _ASM_GENERIC_BITOPS_NON_ATOMIC_H_
>  #define _ASM_GENERIC_BITOPS_NON_ATOMIC_H_
>
> -#include <asm/types.h>
> +#include <asm-generic/bitops/generic-non-atomic.h>
>
> -/**
> - * arch___set_bit - Set a bit in memory
> - * @nr: the bit to set
> - * @addr: the address to start counting from
> - *
> - * Unlike set_bit(), this function is non-atomic and may be reordered.
> - * If it's called on the same region of memory simultaneously, the effect
> - * may be that only one operation succeeds.
> - */
> -static __always_inline void
> -arch___set_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -
> -       *p  |= mask;
> -}
> +#define arch___set_bit generic___set_bit
>  #define __set_bit arch___set_bit
>
> -static __always_inline void
> -arch___clear_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -
> -       *p &= ~mask;
> -}
> +#define arch___clear_bit generic___clear_bit
>  #define __clear_bit arch___clear_bit
>
> -/**
> - * arch___change_bit - Toggle a bit in memory
> - * @nr: the bit to change
> - * @addr: the address to start counting from
> - *
> - * Unlike change_bit(), this function is non-atomic and may be reordered.
> - * If it's called on the same region of memory simultaneously, the effect
> - * may be that only one operation succeeds.
> - */
> -static __always_inline
> -void arch___change_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -
> -       *p ^= mask;
> -}
> +#define arch___change_bit generic___change_bit
>  #define __change_bit arch___change_bit
>
> -/**
> - * arch___test_and_set_bit - Set a bit and return its old value
> - * @nr: Bit to set
> - * @addr: Address to count from
> - *
> - * This operation is non-atomic and can be reordered.
> - * If two examples of this operation race, one can appear to succeed
> - * but actually fail.  You must protect multiple accesses with a lock.
> - */
> -static __always_inline int
> -arch___test_and_set_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -       unsigned long old = *p;
> -
> -       *p = old | mask;
> -       return (old & mask) != 0;
> -}
> +#define arch___test_and_set_bit generic___test_and_set_bit
>  #define __test_and_set_bit arch___test_and_set_bit
>
> -/**
> - * arch___test_and_clear_bit - Clear a bit and return its old value
> - * @nr: Bit to clear
> - * @addr: Address to count from
> - *
> - * This operation is non-atomic and can be reordered.
> - * If two examples of this operation race, one can appear to succeed
> - * but actually fail.  You must protect multiple accesses with a lock.
> - */
> -static __always_inline int
> -arch___test_and_clear_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -       unsigned long old = *p;
> -
> -       *p = old & ~mask;
> -       return (old & mask) != 0;
> -}
> +#define arch___test_and_clear_bit generic___test_and_clear_bit
>  #define __test_and_clear_bit arch___test_and_clear_bit
>
> -/* WARNING: non atomic and it can be reordered! */
> -static __always_inline int
> -arch___test_and_change_bit(unsigned int nr, volatile unsigned long *addr)
> -{
> -       unsigned long mask = BIT_MASK(nr);
> -       unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
> -       unsigned long old = *p;
> -
> -       *p = old ^ mask;
> -       return (old & mask) != 0;
> -}
> +#define arch___test_and_change_bit generic___test_and_change_bit
>  #define __test_and_change_bit arch___test_and_change_bit
>
> -/**
> - * arch_test_bit - Determine whether a bit is set
> - * @nr: bit number to test
> - * @addr: Address to start counting from
> - */
> -static __always_inline int
> -arch_test_bit(unsigned int nr, const volatile unsigned long *addr)
> -{
> -       return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
> -}
> +#define arch_test_bit generic_test_bit
>  #define test_bit arch_test_bit
>
>  #endif /* _ASM_GENERIC_BITOPS_NON_ATOMIC_H_ */
> --
> 2.36.1
>