[ARM] 3210/1: add missing memory barrier helper for NPTL support

Patch from Nicolas Pitre

Strictly speaking, the NPTL kernel helpers are required for pre ARMv6
only.  They are available on ARMv6+ as well for obvious compatibility
reasons.  However there are cases where extra memory barriers are needed
when using an SMP ARMv6 machine but not on pre-ARMv6.

This patch adds a memory barrier kernel helper that glibc can use as
needed for pre-ARMv6 binaries to be forward compatible with an SMP
kernel on ARMv6, as well as the necessary dmb instructions to the
cmpxchg helper.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Acked-by: NDaniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -611,6 +611,47 @@ ENTRY(__switch_to)
 	.globl	__kuser_helper_start
 __kuser_helper_start:
 
+/*
+ * Reference prototype:
+ *
+ *	void __kernel_memory_barrier(void)
+ *
+ * Input:
+ *
+ *	lr = return address
+ *
+ * Output:
+ *
+ *	none
+ *
+ * Clobbered:
+ *
+ *	the Z flag might be lost
+ *
+ * Definition and user space usage example:
+ *
+ *	typedef void (__kernel_dmb_t)(void);
+ *	#define __kernel_dmb (*(__kernel_dmb_t *)0xffff0fa0)
+ *
+ * Apply any needed memory barrier to preserve consistency with data modified
+ * manually and __kuser_cmpxchg usage.
+ *
+ * This could be used as follows:
+ *
+ * #define __kernel_dmb() \
+ *         asm volatile ( "mov r0, #0xffff0fff; mov lr, pc; sub pc, r0, #95" \
+ *	        : : : "lr","cc" )
+ */
+
+__kuser_memory_barrier:				@ 0xffff0fa0
+
+#if __LINUX_ARM_ARCH__ >= 6 && defined(CONFIG_SMP)
+	mcr	p15, 0, r0, c7, c10, 5	@ dmb
+#endif
+	mov	pc, lr
+
+	.align	5
+
 /*
  * Reference prototype:
  *
@@ -642,6 +683,8 @@ __kuser_helper_start:
  * The C flag is also set if *ptr was changed to allow for assembly
  * optimization in the calling code.
  *
+ * Note: this routine already includes memory barriers as needed.
+ *
  * For example, a user space atomic_add implementation could look like this:
  *
  * #define atomic_add(ptr, val) \
@@ -698,10 +741,16 @@ __kuser_cmpxchg:				@ 0xffff0fc0
 
 #else
 
+#ifdef CONFIG_SMP
+	mcr	p15, 0, r0, c7, c10, 5	@ dmb
+#endif
 	ldrex	r3, [r2]
 	subs	r3, r3, r0
 	strexeq	r3, r1, [r2]
 	rsbs	r0, r3, #0
+#ifdef CONFIG_SMP
+	mcr	p15, 0, r0, c7, c10, 5	@ dmb
+#endif
 	mov	pc, lr
 
 #endif