Re: [PATCH 1/2] asm-generic/bitops: Add set_bit_release()

[Gary Guo]

On Wed Oct 21, 2026 at 10:55 AM BST, Philipp Stanner wrote:
> The kernel already has test_bit_acquire(). A set_bit_release(), however,
> was missing so far. It can be expected to be useful for parties that
> need release ordering, but not the stricter full ordering of the other
> bit-setting functions.
>
> Add set_bit_release().
>
> Signed-off-by: Philipp Stanner <phasta@xxxxxxxxxx>
> ---
>  include/asm-generic/bitops/instrumented-atomic.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/include/asm-generic/bitops/instrumented-atomic.h b/include/asm-generic/bitops/instrumented-atomic.h
> index c010d54275e4..fb2d1472fd4d 100644
> --- a/include/asm-generic/bitops/instrumented-atomic.h
> +++ b/include/asm-generic/bitops/instrumented-atomic.h
> @@ -72,6 +72,20 @@ static __always_inline bool test_and_set_bit(long nr, volatile unsigned long *ad
>  	return arch_test_and_set_bit(nr, addr);
>  }
>  
> +/**
> + * set_bit_release - Set a bit with release ordering
> + * @nr: Bit to set
> + * @addr: Address to start counting from
> + *
> + * This is an atomic ordered operation (implied write memory barrier).
> + */
> +static __always_inline void set_bit_release(long nr, volatile unsigned long *addr)
> +{
> +	kcsan_wmb();
> +	instrument_atomic_write(addr + BIT_WORD(nr), sizeof(long));
> +	arch_set_bit(nr, addr);

This is just wrong. There's no code here that gives you the release semantics.

kcsan_wmb() is just an annotation and does nothing. wmb is also not release
semantics as it does not order LOAD->STORE.

Given that set_bit is a relaxed RMW atomic, you can implement set_bit_release
with just

    smp_mb__before_atomic();
    set_bit(nr, addr);

Which should be correct, but is too strong on some architectures (like RISC-V),
becuase it will produce a full MB instead. So we can either add release barriers
(hint: I really like it), or we can add arch_set_bit_release and implement it
per arch; on RISC-V for example it can then be translated to amoor.rl (which is
not being produced by any existing bit ops).c

Best,
Gary

> +}
> +
>  /**
>   * test_and_clear_bit - Clear a bit and return its old value
>   * @nr: Bit to clear