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SubjectRe: Interesting csd deadlock on ARC
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On Tuesday 23 February 2016 03:28 PM, Peter Zijlstra wrote:
> On Tue, Feb 23, 2016 at 10:51:42AM +0530, Vineet Gupta wrote:
>> On Friday 19 February 2016 12:17 PM, Vineet Gupta wrote:
>>> Hi Peter,
>>>
>>> I've been debugging a csd_lock_wait() deadlock on SMP+PREEMPT ARC HS38x2 and it
>>> turned out to be lot more interesting than I'd hoped for. This is stock v4.4
>>>
>>> Trouble starts with an IPI to self which doesn't get delivered as the inter-core
>>> interrupt providing h/w is not capable of IPI to self (which I found as part of
>>> debugging this). Subsequent IPIs from other cores to this core get elided as well
>>> due to the IPI coalescing optimization in arch/arc/kernel/smp.c: ipi_send_msg_one()
>>>
>>> There are ways to use a different h/w mechanism to solve the trigger issue and I'd
>>> hoped to just implement arch_irq_work_raise().
>
> Yes, there are other architectures that use other means for self-IPI,
> IIRC PowerPC has to program their timer in the past to generate a local
> interrupt.
>
>>> But the trouble is the call stack
>>> for this issue: IPI to self is triggered from
>>>
>>> sys_sched_setscheduler
>>> __balance_callback
>>> pull_rt_task
>>> irq_work_queue_on <-- called with @cpu == self
>>>
>>> Looking into irq_work.c, irq_work_queue() is what is semantically needed,
>>> specifically arch_irq_work_raise() will not be called, which means I need
>>> arch_send_call_function_single_ipi() to be able to IPI to self cpu also. Is that
>>> expected from arch code....
>>
>> What I actually meant was is it OK for irq_work_queue_on() to be called locally
>> (is this a sched bug/optimization(. Further if it is OK to be called, does it need
>> to do behave more like irq_work_queue() i.e. call arch_irq_work_raise() or
>> arch_send_call_function_single_ipi() is expected to handle sending IPI to self !
>
> Right, so I'm not actually sure we started out with this requirement.
> But you're not the first to run into this, see:
>
> lkml.kernel.org/r/CAJZ5v0gLankSuziQq25qTCyNqeOX43yD9jnJu_XXwbdyajfmKg@mail.gmail.com
>
> Initially I think irq_work_queue_on() was only used remotely, but I
> think it makes sense to allow the current cpu, esp. since people seem to
> be using it like that.

So it seems Russell's questions in the thread above stands still. IMO we need to
massage irq_work_queue_on() to handle the case of called for local cpu. This will
automatically take care of CONFIG_SMP kernel running on UP hardware.

>
> Now the distinct difference between arch_irq_work_raise() and
> arch_send_call_function_single_ipi() is that arch_irq_work_raise()
> should be NMI-safe.
>
> So on x86 it has to be extra careful about the lapic state, whereas the
> regular IPI code doesn't.
>
> I seem to have forgotten the status of NMIs on ARC, but this is
> something to make a note of.
>

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