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

Mark Rutland
2026-03-30T13:33:39Z

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@kernel.org>
> Reviewed-by: André Almeida <andrealmeid@igalia.com>
> ---
> 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)
>