Messages in this thread | | | Date | Mon, 9 Apr 2018 12:26:00 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH] arm64: avoid race condition issue in dump_backtrace |
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On Sun, Apr 08, 2018 at 03:58:48PM +0800, Ji.Zhang wrote: > Yes, I see where the loop is, I have missed that the loop may cross > different stacks. > Define a nesting order and check against is a good idea, and it can > resolve the issue exactly, but as you mentioned before, we have no idea > how to handle with overflow and sdei stack, and the nesting order is > strongly related with the scenario of the stack, which means if someday > we add another stack, we should consider the relationship of the new > stack with other stacks. From the perspective of your experts, is that > suitable for doing this in unwind? > > Or could we just find some way easier but not so accurate, eg. > Proposal 1: > When we do unwind and detect that the stack spans, record the last fp of > previous stack and next time if we get into the same stack, compare it > with that last fp, the new fp should still smaller than last fp, or > there should be potential loop. > For example, when we unwind from irq to task, we record the last fp in > irq stack such as last_irq_fp, and if it unwind task stack back to irq > stack, no matter if it is the same irq stack with previous, just let it > go and compare the new irq fp with last_irq_fp, although the process may > be wrong since from task stack it could not unwind to irq stack, but the > whole process will eventually stop.
I agree that saving the last fp per-stack could work.
> Proposal 2: > So far we have four types of stack: task, irq, overflow and sdei, could > we just assume that the MAX number of stack spanning is just 3 > times?(task->irq->overflow->sdei or task->irq->sdei->overflow), if yes, > we can just check the number of stack spanning when we detect the stack > spans.
I also agree that counting the number of stack transitions will prevent an inifinite loop, even if less accurately than proposal 1.
I don't have a strong preference either way.
Thanks, Mark.
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