On Fri, Sep 18, 2015 at 04:03:02PM +0100, Catalin Marinas wrote: > On Fri, Sep 18, 2015 at 09:57:56PM +0900, Jungseok Lee wrote: > > On Sep 18, 2015, at 1:21 AM, Catalin Marinas wrote: > > > So, without any better suggestion for current_thread_info(), I'm giving > > > up the idea of using SPSel == 0 in the kernel. I'll look at your patch > > > in more detail. BTW, I don't think we need the any count for the irq > > > stack as we don't re-enter the same IRQ stack. > > > > Another interrupt could come in since IRQ is enabled when handling softirq > > according to the following information which are self-evident. > > > > (Am I missing something?) > > No. I had the wrong impression that we switch to the softirqd stack for > softirqs but you are right, if we run them on the same stack the IRQs > are enabled and they can be re-entered before we return from this > exception handler. > > I've seen other architectures implementing a dedicated softirq stack but > for now we should just re-use the current IRQ stack. > > > In my first approach using SPSel = 0, current_thread_info function was > > inefficient > > in order to handle this case correctly. > > I agree. And we don't have any other scratch register left as we use > tpidr_el1 for per-cpu areas. > > > BTW, in this context, it is only meaningful to decide whether a current > > interrupt > > is re-enterrant or not. Its actual value is not important, but I could not > > figure > > out a better implementation than this one yet. Any suggestions are welcome! > > James' idea of checking the lower SP bits instead of a count may work.
Another thought (it seems that x86 does something similar): we know the IRQ stack is not re-entered until interrupts are enabled in __do_softirq. If we enable __ARCH_HAS_DO_SOFTIRQ, we can implement an arm64-specific do_softirq_own_stack() which increments a counter before calling __do_softirq. The difference from your patch is that irq_stack_entry only reads such counter, doesn't need to write it. Yet another idea is to reserve some space in the lower address part of the stack with a "stack type" information. It still requires another read, so I think the x86 approach is probably better. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/