[patch V5 06/16] uaccess: Provide unsafe_atomic_store_release_user()

[Thomas Gleixner <tglx@kernel.org>]

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>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
---
V4: Rename it really ....
    Add a barrier when TSO=y
V3: Rename to CONFIG_ARCH_MEMORY_ORDER_TSO - Peter
V2: New patch
---
 arch/Kconfig            |    4 ++++
 include/linux/uaccess.h |   11 +++++++++++
 2 files changed, 15 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 (TSO)
+config ARCH_MEMORY_ORDER_TSO
+	bool
+
 config HAVE_ASM_MODVERSIONS
 	bool
 	help
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -649,6 +649,17 @@ 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_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)