linux-kernel - [tip:sched/core] sched/preempt, arm/futex: Disable preemption in UP futex_atomic_op

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Date:	Tue, 19 May 2015 00:21:23 -0700
From:	tip-bot for David Hildenbrand <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	mingo@...nel.org, hpa@...or.com, torvalds@...ux-foundation.org,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	dahi@...ux.vnet.ibm.com, peterz@...radead.org
Subject: [tip:sched/core] sched/preempt, arm/futex:
  Disable preemption in UP futex_atomic_op_inuser() explicitly

Commit-ID:  388b0e0adbc98a1b12a077dc92851a3ce016db42
Gitweb:     http://git.kernel.org/tip/388b0e0adbc98a1b12a077dc92851a3ce016db42
Author:     David Hildenbrand <dahi@...ux.vnet.ibm.com>
AuthorDate: Mon, 11 May 2015 17:52:16 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
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@...utronix.de>
Signed-off-by: David Hildenbrand <dahi@...ux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: David.Laight@...LAB.COM
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: airlied@...ux.ie
Cc: akpm@...ux-foundation.org
Cc: benh@...nel.crashing.org
Cc: bigeasy@...utronix.de
Cc: borntraeger@...ibm.com
Cc: daniel.vetter@...el.com
Cc: heiko.carstens@...ibm.com
Cc: herbert@...dor.apana.org.au
Cc: hocko@...e.cz
Cc: hughd@...gle.com
Cc: mst@...hat.com
Cc: paulus@...ba.org
Cc: ralf@...ux-mips.org
Cc: schwidefsky@...ibm.com
Cc: yang.shi@...driver.com
Link: http://lkml.kernel.org/r/1431359540-32227-12-git-send-email-dahi@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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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