Re: [patch V3 05/14] uaccess: Provide unsafe_atomic_store_release_user()

[Mark Rutland]

On Mon, Mar 30, 2026 at 02:02:25PM +0200, Thomas Gleixner wrote:
> The upcoming support for unlocking robust futexes in the kernel requires
> store release semantics. Syscalls do not imply memory ordering on all
> architectures so the unlock operation requires a barrier.
> 
> This barrier can be avoided when stores imply release like on x86.
> 
> Provide a generic version with a smp_mb() before the unsafe_put_user(),
> which can be overridden by architectures.
> 
> Provide also a ARCH_MEMORY_ORDER_TOS Kconfig option, which can be selected
> by architectures with Total Store Order (TSO), where store implies release,
> so that the smp_mb() in the generic implementation can be avoided.
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Reviewed-by: André Almeida <andrealmeid@xxxxxxxxxx>
> ---
> V3: Rename to CONFIG_ARCH_MEMORY_ORDER_TSO - Peter

Looks like this was missed, or applied incompletely? There are a number
of uses of "TOS" rather than "TSO", above, below, and in subsequent
patches.

Mark.

> V2: New patch
> ---
>  arch/Kconfig            |    4 ++++
>  include/linux/uaccess.h |    9 +++++++++
>  2 files changed, 13 insertions(+)
> 
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -403,6 +403,10 @@ config ARCH_32BIT_OFF_T
>  config ARCH_32BIT_USTAT_F_TINODE
>  	bool
>  
> +# Selected by architectures with Total Store Order (TOS)
> +config ARCH_MEMORY_ORDER_TOS
> +	bool
> +
>  config HAVE_ASM_MODVERSIONS
>  	bool
>  	help
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -644,6 +644,15 @@ static inline void user_access_restore(u
>  #define user_read_access_end user_access_end
>  #endif
>  
> +#ifndef unsafe_atomic_store_release_user
> +# define unsafe_atomic_store_release_user(val, uptr, elbl)	\
> +	do {							\
> +		if (!IS_ENABLED(CONFIG_ARCH_MEMORY_ORDER_TOS))	\
> +			smp_mb();				\
> +		unsafe_put_user(val, uptr, elbl);		\
> +	} while (0)
> +#endif
> +
>  /* Define RW variant so the below _mode macro expansion works */
>  #define masked_user_rw_access_begin(u)	masked_user_access_begin(u)
>  #define user_rw_access_begin(u, s)	user_access_begin(u, s)
>