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Why nested interrupt corrupt Link Register?

Harri Wijaya
Note: This was originally posted on 13th June 2012 at http://forums.arm.com

hi,
Sorry for this basic question.

I find hard to understand why the Link Register can be corrupted in this below scenario:
1. IRQ interrupt occur.
2. IRQ ISR call a function foo(). (assume inside ISR, the IRQ interrupt is reenabled)
3. foo() is interrupted by another IRQ interrupt.

From what I understand:

*During (1):

STACK content:
some general purpose registers
LR_irq_1 (let call like that to indicate that the content is point to address of interrupted function by scenario-1)

LR_irq = address-1 (see LR_irq_1)

*During (2):

STACK content:
some general purpose registers
LR_irq_2
some general purpose registers
LR_irq_1

LR_irq = address-2 (address of a line within ISR)

*During (3):

STACK content:
some general purpose registers
LR_irq_3
some general purpose registers
LR_irq_2
some general purpose registers
LR_irq_1

LR_irq = address-3 (address of a line within foo() where second IRQ interrupt occur)


Assuming that we have enough IRQ stack size, then from above context, seem no reason for LR to be corrupted.

Any body could help me explain what causing LR to be corrupted?

Thanks!
Parents
  • Note: This was originally posted on 15th June 2012 at http://forums.arm.com

    Ouch!, yes that could happen. Although the possibility is lower (only one place is vulnerable) than in "leaf function" (where any place is vulnerable). But if LR is used as temporary, then "game over" (borrow your term ).
    Nice analysis, scott! Really improve my understanding. :)

    So as suggested by many books/sources, we can rid off all that problems by switching to SYS mode before executing interrupt handler. (does it really rid off ALL the problems? any "hole"?)
Reply
  • Note: This was originally posted on 15th June 2012 at http://forums.arm.com

    Ouch!, yes that could happen. Although the possibility is lower (only one place is vulnerable) than in "leaf function" (where any place is vulnerable). But if LR is used as temporary, then "game over" (borrow your term ).
    Nice analysis, scott! Really improve my understanding. :)

    So as suggested by many books/sources, we can rid off all that problems by switching to SYS mode before executing interrupt handler. (does it really rid off ALL the problems? any "hole"?)
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