futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test

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>

4 files changed, 36 insertions, 13 deletions

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 65a07750f4f9..bb74b21f007a 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -117,6 +117,7 @@ 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_LZ4
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 009a797dd242..d56cb03c1b49 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1387,6 +1387,13 @@ 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 44a1261cb9ff..5d17e3a83f8c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -157,7 +157,9 @@
  * enqueue.
  */
 
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
 int __read_mostly futex_cmpxchg_enabled;
+#endif
 
 /*
  * Futex flags used to encode options to functions and preserve them across
@@ -2843,9 +2845,28 @@ 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;
+
+        /*
+         * This will fail and we want it. Some arch implementations do
+         * runtime detection of the futex_atomic_cmpxchg_inatomic()
+         * functionality. We want to know that before we call in any
+         * of the complex code paths. Also we want to prevent
+         * registration of robust lists in that case. NULL is
+         * guaranteed to fault and we get -EFAULT on functional
+         * implementation, the non-functional ones will return
+         * -ENOSYS.
+         */
+        if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
+                futex_cmpxchg_enabled = 1;
+#endif
+}
+
+static int __init futex_init(void)
+{
         unsigned int futex_shift;
         unsigned long i;
 
@@ -2861,18 +2882,8 @@ static int __init futex_init(void)
                                                &futex_shift, NULL,
                                                futex_hashsize, futex_hashsize);
         futex_hashsize = 1UL << futex_shift;
-        /*
-         * This will fail and we want it. Some arch implementations do
-         * runtime detection of the futex_atomic_cmpxchg_inatomic()
-         * functionality. We want to know that before we call in any
-         * of the complex code paths. Also we want to prevent
-         * registration of robust lists in that case. NULL is
-         * guaranteed to fault and we get -EFAULT on functional
-         * implementation, the non-functional ones will return
-         * -ENOSYS.
-         */
-        if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
-                futex_cmpxchg_enabled = 1;
+
+        futex_detect_cmpxchg();
 
         for (i = 0; i < futex_hashsize; i++) {
                 plist_head_init(&futex_queues[i].chain);