[patch v2 04/11] uaccess: Provide unsafe_atomic_store_release_user()

[Thomas Gleixner]

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_STORE_IMPLIES_RELEASE Kconfig option, which can be
selected by architectures where store implies release, so that the smp_mb()
in the generic implementation can be avoided.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
---
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 when plain stores have release semantics
+config ARCH_STORE_IMPLIES_RELEASE
+	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_STORE_IMPLIES_RELEASE))	\
+			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)