linux-kernel - [PATCH 3.11 04/46] futex: Allow architectures to skip futex_atomic_cmpxchg

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Message-Id: <1397644322-13905-5-git-send-email-luis.henriques@canonical.com>
Date:	Wed, 16 Apr 2014 11:31:20 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Finn Thain <fthain@...egraphics.com.au>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.11 04/46] futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test

3.11.10.8 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Heiko Carstens <heiko.carstens@...ibm.com>

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@...ibm.com>
Cc: Finn Thain <fthain@...egraphics.com.au>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
[ Geert: backported version for v3.10..v3.13 ]
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 arch/s390/Kconfig     |  1 +
 include/linux/futex.h |  4 ++++
 init/Kconfig          |  7 +++++++
 kernel/futex.c        | 14 ++++++++++++--
 4 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 8a4cae7..f443713 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_LZ4
diff --git a/include/linux/futex.h b/include/linux/futex.h
index b0d95ca..6435f46 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 fed81b5..390ff19 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1402,6 +1402,13 @@ 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 221a58f..15a7d724 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -68,7 +68,9 @@
 
 #include "rtmutex_common.h"
 
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
 int __read_mostly futex_cmpxchg_enabled;
+#endif
 
 #define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
 
@@ -2730,10 +2732,10 @@ 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
@@ -2747,6 +2749,14 @@ 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);
-- 
1.9.1

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