ARM: mvebu: Fix bug in coherency fabric low level init function

When adding CPU to the SMP group and enabling the coherency on this
CPU we must protect the register access.
The previous implementation claims to be atomic but doesn't provide
any protection against parallel access to the coherency fabric control
and configuration registers.

This patch fixes this by using the ldrex and strex mechanism.
This method should be used in all accesses to those registers.

[gregory.clement@free-electrons.com: fixed the commit's topic]
Signed-off-by: Nadav Haklai <nadavh@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

arch/arm/mach-mvebu/coherency_ll.S

@@ -32,15 +32,21 @@ ENTRY(ll_set_cpu_coherent)
 
 	/* Add CPU to SMP group - Atomic */
 	add	r3, r0, #ARMADA_XP_CFB_CTL_REG_OFFSET
-	ldr	r2, [r3]
+1:
+	ldrex	r2, [r3]
 	orr	r2, r2, r1
-	str	r2, [r3]
+	strex 	r0, r2, [r3]
+	cmp	r0, #0
+	bne 1b
 
 	/* Enable coherency on CPU - Atomic */
-	add	r3, r0, #ARMADA_XP_CFB_CFG_REG_OFFSET
-	ldr	r2, [r3]
+	add	r3, r3, #ARMADA_XP_CFB_CFG_REG_OFFSET
+1:
+	ldrex	r2, [r3]
 	orr	r2, r2, r1
-	str	r2, [r3]
+	strex	r0, r2, [r3]
+	cmp	r0, #0
+	bne 1b
 
 	dsb