arm64: let the core code deal with preempt_count

Commit f27dde8d (sched: Add NEED_RESCHED to the preempt_count)
introduced the use of bit 31 in preempt_count for obscure scheduling
purposes.

This causes interrupts taken from EL0 to hit the (open coded) BUG when
this flag is flipped while handling the interrupt (we compare the
values before and after, and kill the kernel if they are different).

The fix is to stop messing with the preempt count entirely, as this
is already being dealt with in the generic code (irq_enter/irq_exit).

Tested on a dual A53 FPGA running cyclictest.
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -309,15 +309,12 @@ el1_irq:
 #ifdef CONFIG_TRACE_IRQFLAGS
 	bl	trace_hardirqs_off
 #endif
-#ifdef CONFIG_PREEMPT
-	get_thread_info tsk
-	ldr	w24, [tsk, #TI_PREEMPT]		// get preempt count
-	add	w0, w24, #1			// increment it
-	str	w0, [tsk, #TI_PREEMPT]
-#endif
+
 	irq_handler
+
 #ifdef CONFIG_PREEMPT
-	str	w24, [tsk, #TI_PREEMPT]		// restore preempt count
+	get_thread_info tsk
+	ldr	w24, [tsk, #TI_PREEMPT]		// restore preempt count
 	cbnz	w24, 1f				// preempt count != 0
 	ldr	x0, [tsk, #TI_FLAGS]		// get flags
 	tbz	x0, #TIF_NEED_RESCHED, 1f	// needs rescheduling?
@@ -507,22 +504,10 @@ el0_irq_naked:
 #ifdef CONFIG_TRACE_IRQFLAGS
 	bl	trace_hardirqs_off
 #endif
-	get_thread_info tsk
-#ifdef CONFIG_PREEMPT
-	ldr	w24, [tsk, #TI_PREEMPT]		// get preempt count
-	add	w23, w24, #1			// increment it
-	str	w23, [tsk, #TI_PREEMPT]
-#endif
+
 	irq_handler
-#ifdef CONFIG_PREEMPT
-	ldr	w0, [tsk, #TI_PREEMPT]
-	str	w24, [tsk, #TI_PREEMPT]
-	cmp	w0, w23
-	b.eq	1f
-	mov	x1, #0
-	str	x1, [x1]			// BUG
-1:
-#endif
+	get_thread_info tsk
+
 #ifdef CONFIG_TRACE_IRQFLAGS
 	bl	trace_hardirqs_on
 #endif