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uaccess: Provide unsafe_atomic_store_release_user()

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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_TSO 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.

If that is set a barrier() is used instead of smp_mb(), which is not
required for the use case at hand, but makes it future proof for other
usage to prevent the compiler from reordering.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.513181528@kernel.org
This commit is contained in:
Thomas Gleixner 2026-06-02 11:09:42 +02:00 committed by Peter Zijlstra
parent 2cb5251d3d
commit 6149fc36c0
2 changed files with 15 additions and 0 deletions
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4
arch/Kconfig
View File

@ -403,6 +403,10 @@ config ARCH_32BIT_OFF_T
config ARCH_32BIT_USTAT_F_TINODE
bool
# Selected by architectures with Total Store Order (TSO)
config ARCH_MEMORY_ORDER_TSO
bool
config HAVE_ASM_MODVERSIONS
bool
help

11
include/linux/uaccess.h
View File

@ -649,6 +649,17 @@ static inline void user_access_restore(unsigned long flags) { }
#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_TSO)) \
smp_mb(); \
else \
barrier(); \
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)