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

Thomas Gleixner
2026-03-30T12:02:29Z

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
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)