[tip:sched/core] sched/preempt, arm/futex: Disable preemption in UP futex_atomic_op_inuser() explicitly

[tip-bot for David Hildenbrand]

Commit-ID:  388b0e0adbc98a1b12a077dc92851a3ce016db42
Gitweb:     http://git.kernel.org/tip/388b0e0adbc98a1b12a077dc92851a3ce016db42
Author:     David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
AuthorDate: Mon, 11 May 2015 17:52:16 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 19 May 2015 08:39:17 +0200

sched/preempt, arm/futex: Disable preemption in UP futex_atomic_op_inuser() explicitly

The !CONFIG_SMP implementation of futex_atomic_op_inuser() seems to rely
on disabled preemption to guarantee mutual exclusion.

>From commit e589ed23dd27 ("[ARM] 5218/1: arm: improved futex support"):

    "For UP it's enough to disable preemption to ensure mutual exclusion..."

>From the code itself:

    "!SMP, we can work around lack of atomic ops by disabling preemption"

Let's make this explicit, to prepare for pagefault_disable() not
touching preemption anymore.

Reviewed-and-tested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: David.Laight@xxxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: airlied@xxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx
Cc: benh@xxxxxxxxxxxxxxxxxxx
Cc: bigeasy@xxxxxxxxxxxxx
Cc: borntraeger@xxxxxxxxxx
Cc: daniel.vetter@xxxxxxxxx
Cc: heiko.carstens@xxxxxxxxxx
Cc: herbert@xxxxxxxxxxxxxxxxxxx
Cc: hocko@xxxxxxx
Cc: hughd@xxxxxxxxxx
Cc: mst@xxxxxxxxxx
Cc: paulus@xxxxxxxxx
Cc: ralf@xxxxxxxxxxxxxx
Cc: schwidefsky@xxxxxxxxxx
Cc: yang.shi@xxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1431359540-32227-12-git-send-email-dahi@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/arm/include/asm/futex.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index 255bfd1..5eed828 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -127,7 +127,10 @@ futex_atomic_op_inuser (int encoded_op, u32 __user *uaddr)
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
 		return -EFAULT;
 
-	pagefault_disable();	/* implies preempt_disable() */
+#ifndef CONFIG_SMP
+	preempt_disable();
+#endif
+	pagefault_disable();
 
 	switch (op) {
 	case FUTEX_OP_SET:
@@ -149,7 +152,10 @@ futex_atomic_op_inuser (int encoded_op, u32 __user *uaddr)
 		ret = -ENOSYS;
 	}
 
-	pagefault_enable();	/* subsumes preempt_enable() */
+	pagefault_enable();
+#ifndef CONFIG_SMP
+	preempt_enable();
+#endif
 
 	if (!ret) {
 		switch (cmp) {
--
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