Re: [PATCH v3 1/5] lib: Add two-byte cmpxchg emulation function

[David Laight <david.laight.linux@gmail.com>]

On Thu, 17 Sep 2026 16:38:26 +0000
Bradley Morgan <brads@mainlining.org> wrote:

> cmpxchg_emu_u8() emulates one-byte cmpxchg() in terms of four-byte
> cmpxchg() for the architectures lacking native one-byte atomics.
> The same architectures also lack native two-byte cmpxchg(), where
> such an operation is not supported and either fails to compile via
> BUILD_BUG() or fails to link, because the bad pointer sentinels
> these architectures declare are never defined.
> 
> Add cmpxchg_emu_u16(), the two-byte sibling. It reads the enclosing
> word with READ_ONCE(), splices the two target bytes through a union
> and loops on cmpxchg() of the full word until the compare succeeds.
> Like cmpxchg_emu_u8() it is fully ordered.
> 
> Unlike cmpxchg_emu_u8() it casts the old and new values to u16
> internally and returns unsigned long, taking the old and new values
> as unsigned long, per the suggestion from David Laight. The switch
> statements in the architecture macros instantiate every size case,
> so a cmpxchg() on a pointer type checks the two-byte case as well,
> and a u16 parameter or return would make the macro casts and return
> conversions warn there. With unsigned long parameters and return the
> call sites need no narrowing casts, pointer exchanges compile warning
> free, and the function still compares and returns exactly the 16 bits
> the caller asked for, which matches the hardware cmpxchg r16
> behaviour where a 16-bit compare only looks at the low 16 bits of
> the register.
> 
> The Kconfig symbol gating this file is renamed from
> ARCH_NEED_CMPXCHG_1_EMU to ARCH_NEED_CMPXCHG_1_2_EMU, as it now
> selects both the one-byte and the two-byte emulation.
> 
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Suggested-by: David Laight <david.laight.linux@gmail.com>
> Signed-off-by: Bradley Morgan <brads@mainlining.org>
> ---
...
> diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
> index 27f6f97cb60d..ac1c84383887 100644
> --- a/lib/cmpxchg-emu.c
> +++ b/lib/cmpxchg-emu.c
> @@ -1,10 +1,11 @@
>  // SPDX-License-Identifier: GPL-2.0+
>  /*
> - * Emulated 1-byte cmpxchg operation for architectures lacking direct
> - * support for this size.  This is implemented in terms of 4-byte cmpxchg
> - * operations.
> + * Emulated 1-byte and 2-byte cmpxchg operations for architectures lacking
> + * direct support for these sizes.  These are implemented in terms of
> + * 4-byte cmpxchg operations.
>   *
> - * Copyright (C) 2024 Paul E. McKenney.
> + * Copyright (C) 2024 Paul E. McKenney <paulmck@kernel.org>
> + * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
>   */
>  
>  #include <linux/types.h>
> @@ -40,6 +41,35 @@ uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
>  		instrument_atomic_read_write(p, 1);
>  		ret = data_race(cmpxchg(p32, old32.w, new32.w)); // Overridden above.
>  	} while (ret != old32.w);
> -	return old;
> +	return (u16)old;
>  }
>  EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);

Where did the (u16) cast come from?
If you want to mask 'old' to 8 bits it would be more reasonable to change
the function prototype.
(Although I can never remember whether the caller or called code is
responsible for masking the value.)

David