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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@linux.vnet.ibm.com>
AuthorDate: Mon, 11 May 2015 17:52:16 +0200
Committer: Ingo Molnar <mingo@kernel.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@linutronix.de>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David.Laight@ACULAB.COM
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: airlied@linux.ie
Cc: akpm@linux-foundation.org
Cc: benh@kernel.crashing.org
Cc: bigeasy@linutronix.de
Cc: borntraeger@de.ibm.com
Cc: daniel.vetter@intel.com
Cc: heiko.carstens@de.ibm.com
Cc: herbert@gondor.apana.org.au
Cc: hocko@suse.cz
Cc: hughd@google.com
Cc: mst@redhat.com
Cc: paulus@samba.org
Cc: ralf@linux-mips.org
Cc: schwidefsky@de.ibm.com
Cc: yang.shi@windriver.com
Link: http://lkml.kernel.org/r/1431359540-32227-12-git-send-email-dahi@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.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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