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 <[e-mail]>
Cc: Peter Zijlstra (Intel) <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Michel Lespinasse <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/include/asm/futex.h

···
 	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) {
···
 	}
 
 	pagefault_enable();
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_enable();
+#endif
 
 	if (!ret) {
 		switch (cmp) {
···
 	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__(
···
 
 	smp_mb();
 
+#ifndef CONFIG_ARC_HAS_LLSC
+	preempt_enable();
+#endif
 	*uval = existval;
 	return ret;
 }