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ARC: !PREEMPT: Ensure Return to kernel mode is IRQ safe
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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 <[email protected]>
Cc: <[email protected]>   #3.10, 3.12, 3.13, 3.14
Cc: Anton Kolesov <[email protected]>
Cc: Francois Bedard <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Vineet Gupta authored and torvalds committed Apr 30, 2014
1 parent 71dc96e commit 8aa9e85
Showing 1 changed file with 5 additions and 3 deletions.
8 changes: 5 additions & 3 deletions arch/arc/kernel/entry.S
Original file line number Diff line number Diff line change
Expand Up @@ -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]
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