linux-kernel - Re: Interesting csd deadlock on ARC

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Date:	Wed, 24 Feb 2016 10:21:25 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Russell King <linux@....linux.org.uk>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Marc Zyngier <marc.zyngier@....com>,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Noam Camus" <noamc@...hip.com>,
	arcml <linux-snps-arc@...ts.infradead.org>
Subject: Re: Interesting csd deadlock on ARC

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@...l.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.
>