[3.13.y.z extended stable] Patch "ARC: !PREEMPT: Ensure Return to kernel mode is IRQ safe" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 7072773c86abda646f5a51e438d20c8a5d5a04ef Mon Sep 17 00:00:00 2001
From: Vineet Gupta <Vineet.Gupta1 at synopsys.com>
Date: Wed, 30 Apr 2014 15:26:45 +0530
Subject: 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 at synopsys.com>
Cc: Anton Kolesov <Anton.Kolesov at synopsys.com>
Cc: Francois Bedard <Francois.Bedard at synopsys.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 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 47d09d0..6e8f83a 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]
--
1.9.1