linux-kernel - [PATCH 5/4] ARC: ensure futex ops are atomic in !LLSC config

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Date:	Thu, 6 Aug 2015 19:16:20 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
CC:	lkml <linux-kernel@...r.kernel.org>,
	Vineet Gupta <Vineet.Gupta1@...opsys.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michel Lespinasse <walken@...gle.com>
Subject: [PATCH 5/4] ARC: ensure futex ops are atomic in !LLSC config

W/o hardware assisted atomic r-m-w the best we can do is to disable
preemption.

Cc: David Hildenbrand <dahi@...ux.vnet.ibm.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Michel Lespinasse <walken@...gle.com>
Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
---
 arch/arc/include/asm/futex.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/arc/include/asm/futex.h b/arch/arc/include/asm/futex.h
index 0ea8bcc7b846..8f449982523b 100644
--- a/arch/arc/include/asm/futex.h
+++ b/arch/arc/include/asm/futex.h
@@ -87,6 +87,9 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(int)))
 		return -EFAULT;
 
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_disable();	/* to guarantee atomic r-m-w of futex op */
+#endif
 	pagefault_disable();
 
 	switch (op) {
@@ -111,6 +114,9 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
 	}
 
 	pagefault_enable();
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_enable();
+#endif
 
 	if (!ret) {
 		switch (cmp) {
@@ -153,6 +159,9 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 expval,
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
 		return -EFAULT;
 
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_disable();	/* to guarantee atomic r-m-w of futex op */
+#endif
 	smp_mb();
 
 	__asm__ __volatile__(
@@ -182,6 +191,9 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 expval,
 
 	smp_mb();
 
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_enable();
+#endif
 	*uval = existval;
 	return ret;
 }
-- 
1.9.1

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