uaccess: Provide unsafe_atomic_store_release_user()

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

diff --git a/arch/Kconfig b/arch/Kconfig
index e86880045158e4b3439645590e3c8f56f2a5d435..83d362fca38cdf97eca1055e514a30b893c8dc42 100644 (file)
--- 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 (TSO)
+config ARCH_MEMORY_ORDER_TSO
+       bool
+
 config HAVE_ASM_MODVERSIONS
        bool
        help

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 56328601218c5a7c9a23f3c8e3c2dcd764720fbd..c6bd20065a5d77708053f50978ae32b070723ae3 100644 (file)
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -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)