ARC: !PREEMPT: Ensure Return to kernel mode is IRQ safe

There was a very small race window where resume to kernel mode from a
Exception Path (or pure kernel mode which is true for most of ARC
exceptions anyways), was not disabling interrupts in restore_regs,
clobbering the exception regs

Anton found the culprit call flow (after many sleepless nights)

| 1. we got a Trap from user land
| 2. started to service it.
| 3. While doing some stuff on user-land memory (I think it is padzero()),
|     we got a DataTlbMiss
| 4. On return from it we are taking "resume_kernel_mode" path
| 5. NEED_RESHED is not set, so we go to "return from exception" path in
|     restore regs.
| 6. there seems to be IRQ happening

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: <stable@vger.kernel.org>   #3.10, 3.12, 3.13, 3.14
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Francois Bedard <Francois.Bedard@synopsys.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index 819dd5f7eb05..29b82adbf0b4 100644
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -614,11 +614,13 @@ resume_user_mode_begin:
 
 resume_kernel_mode:
 
-#ifdef CONFIG_PREEMPT
-
-        ; This is a must for preempt_schedule_irq()
+        ; Disable Interrupts from this point on
+        ; CONFIG_PREEMPT: This is a must for preempt_schedule_irq()
+        ; !CONFIG_PREEMPT: To ensure restore_regs is intr safe
         IRQ_DISABLE     r9
 
+#ifdef CONFIG_PREEMPT
+
         ; Can't preempt if preemption disabled
         GET_CURR_THR_INFO_FROM_SP   r10
         ld  r8, [r10, THREAD_INFO_PREEMPT_COUNT]