[PATCH 3.12 141/182] ARC: !PREEMPT: Ensure Return to kernel mode is IRQ safe

[Jiri Slaby]

From: Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

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@xxxxxxxxxxxx>
Cc: Anton Kolesov <Anton.Kolesov@xxxxxxxxxxxx>
Cc: Francois Bedard <Francois.Bedard@xxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 arch/arc/kernel/entry.S | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/arc/kernel/entry.S b/arch/arc/kernel/entry.S
index b908dde8a331..15588b0611e9 100644
--- a/arch/arc/kernel/entry.S
+++ b/arch/arc/kernel/entry.S
@@ -610,11 +610,13 @@ 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]
-- 
1.9.3

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