futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test

commit 03b8c7b623c80af264c4c8d6111e5c6289933666 upstream.

If an architecture has futex_atomic_cmpxchg_inatomic() implemented and there
is no runtime check necessary, allow to skip the test within futex_init().

This allows to get rid of some code which would always give the same result,
and also allows the compiler to optimize a couple of if statements away.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Finn Thain <fthain@telegraphics.com.au>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[geert: Backported to v3.10..v3.13]
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 24 insertions, 2 deletions

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 97dcbea97a1c..d8d6eeca56b0 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -116,6 +116,7 @@ config S390
         select HAVE_FUNCTION_GRAPH_TRACER
         select HAVE_FUNCTION_TRACER
         select HAVE_FUNCTION_TRACE_MCOUNT_TEST
+        select HAVE_FUTEX_CMPXCHG if FUTEX
         select HAVE_KERNEL_BZIP2
         select HAVE_KERNEL_GZIP
         select HAVE_KERNEL_LZMA
diff --git a/include/linux/futex.h b/include/linux/futex.h
index b0d95cac826e..6435f46d6e13 100644
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -55,7 +55,11 @@ union futex_key {
 #ifdef CONFIG_FUTEX
 extern void exit_robust_list(struct task_struct *curr);
 extern void exit_pi_state_list(struct task_struct *curr);
+#ifdef CONFIG_HAVE_FUTEX_CMPXCHG
+#define futex_cmpxchg_enabled 1
+#else
 extern int futex_cmpxchg_enabled;
+#endif
 #else
 static inline void exit_robust_list(struct task_struct *curr)
 {
diff --git a/init/Kconfig b/init/Kconfig
index 2d9b83104dcf..5d6febaea56d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1365,6 +1365,13 @@ config FUTEX
           support for "fast userspace mutexes".  The resulting kernel may not
           run glibc-based applications correctly.
 
+config HAVE_FUTEX_CMPXCHG
+        bool
+        help
+          Architectures should select this if futex_atomic_cmpxchg_inatomic()
+          is implemented and always working. This removes a couple of runtime
+          checks.
+
 config EPOLL
         bool "Enable eventpoll support" if EXPERT
         default y
diff --git a/kernel/futex.c b/kernel/futex.c
index a283b3041072..3bc18bf48d0c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -67,7 +67,9 @@
 
 #include "rtmutex_common.h"
 
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
 int __read_mostly futex_cmpxchg_enabled;
+#endif
 
 #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
 
@@ -2729,10 +2731,10 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
         return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
 }
 
-static int __init futex_init(void)
+static void __init futex_detect_cmpxchg(void)
 {
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
         u32 curval;
-        int i;
 
         /*
          * This will fail and we want it. Some arch implementations do
@@ -2746,6 +2748,14 @@ static int __init futex_init(void)
          */
         if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
                 futex_cmpxchg_enabled = 1;
+#endif
+}
+
+static int __init futex_init(void)
+{
+        int i;
+
+        futex_detect_cmpxchg();
 
         for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
                 plist_head_init(&futex_queues[i].chain);