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

commit 8aa9e85adac609588eeec356e5a85059b3b819ba upstream.

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: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Francois Bedard <Francois.Bedard@synopsys.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index 57f38333834b508073583cdfe4becd1fb11a22ee..6f3cd0fb4b54a2411365323cb06f561e09fcf6af 100644 (file)
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -651,11 +651,13 @@ 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]