[AVR32] Fix random segfault with preemption

As explained on:
http://www.avrfreaks.net/index.php?nameÿphpBB2&fileÿewtopic&tS307
If the current process is preempted before it can copy RAR_SUP and
RSR_SUP both register are lost and the process will segfault as soon
as it return from the syscall since the return adress will be
corrupted.

This patch disable IRQ as soon as we enter the syscall path and
reenable them when the copy is done.

In the interrupt handlers, check if we are interrupting the srrf
instruction, if so disable interrupts and return. The interrupt
handler will be re-called immediatly when the interrupts are
reenabled.

After some stressing workload:
 - find / > /dev/null in loop
 - top (in ssh)
 - ping -f avr32

The segfaults are not seen anymore.
Signed-off-by: NPhilippe Rétornaz <philippe.retornaz@epfl.ch>
Signed-off-by: NHaavard Skinnemoen <hskinnemoen@atmel.com>

arch/avr32/kernel/entry-avr32b.S

@@ -159,11 +159,18 @@ handle_vmalloc_miss:
 
 	.section .scall.text,"ax",@progbits
 system_call:
+#ifdef CONFIG_PREEMPT
+	mask_interrupts
+#endif
 	pushm	r12		/* r12_orig */
 	stmts	--sp, r0-lr
-	zero_fp
+
 	mfsr	r0, SYSREG_RAR_SUP
 	mfsr	r1, SYSREG_RSR_SUP
+#ifdef CONFIG_PREEMPT
+	unmask_interrupts
+#endif
+	zero_fp
 	stm	--sp, r0-r1
 
 	/* check for syscall tracing */
@@ -638,6 +645,13 @@ irq_level\level:
 	stmts	--sp,r0-lr
 	mfsr	r8, rar_int\level
 	mfsr	r9, rsr_int\level
+
+#ifdef CONFIG_PREEMPT
+	sub	r11, pc, (. - system_call)
+	cp.w	r11, r8
+	breq	4f
+#endif
+
 	pushm	r8-r9
 
 	mov	r11, sp
@@ -668,6 +682,16 @@ irq_level\level:
 	sub	sp, -4		/* ignore r12_orig */
 	rete
 
+#ifdef CONFIG_PREEMPT
+4:	mask_interrupts
+	mfsr	r8, rsr_int\level
+	sbr	r8, 16
+	mtsr	rsr_int\level, r8
+	ldmts	sp++, r0-lr
+	sub	sp, -4		/* ignore r12_orig */
+	rete
+#endif
+
 2:	get_thread_info	r0
 	ld.w	r1, r0[TI_flags]
 	bld	r1, TIF_CPU_GOING_TO_SLEEP