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Subject: Re: xen/evtchn and forced threaded irq
To:     Julien Grall <julien.grall@arm.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>
Cc:     xen-devel <xen-devel@lists.xenproject.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Dave P Martin <dave.martin@arm.com>
References: <5e256d9a-572c-e01e-7706-407f99245b00@arm.com>
From:   Juergen Gross <jgross@suse.com>
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Date:   Thu, 21 Feb 2019 09:17:10 +0100
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On 19/02/2019 18:31, Julien Grall wrote:
> Hi all,
> 
> I have been looking at using Linux RT in Dom0. Once the guest is started,
> the console is ending to have a lot of warning (see trace below).
> 
> After some investigation, this is because the irq handler will now be threaded.
> I can reproduce the same error with the vanilla Linux when passing the option
> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 that has
> not RT support).
> 
> FWIW, the interrupt for port 6 is used to for the guest to communicate with
> xenstore.
> 
> From my understanding, this is happening because the interrupt handler is now
> run in a thread. So we can have the following happening.
> 
>    Interrupt context            |     Interrupt thread
>                                 |  
>    receive interrupt port 6     |
>    clear the evtchn port        |
>    set IRQF_RUNTHREAD	        |
>    kick interrupt thread        |
>                                 |    clear IRQF_RUNTHREAD
>                                 |    call evtchn_interrupt
>    receive interrupt port 6     |
>    clear the evtchn port        |
>    set IRQF_RUNTHREAD           |
>    kick interrupt thread        |
>                                 |    disable interrupt port 6
>                                 |    evtchn->enabled = false
>                                 |    [....]
>                                 |
>                                 |    *** Handling the second interrupt ***
>                                 |    clear IRQF_RUNTHREAD
>                                 |    call evtchn_interrupt
>                                 |    WARN(...)
> 
> I am not entirely sure how to fix this. I have two solutions in mind:
> 
> 1) Prevent the interrupt handler to be threaded. We would also need to
> switch from spin_lock to raw_spin_lock as the former may sleep on RT-Linux.
> 
> 2) Remove the warning

3) Split the handler (RT-only?) to a non-threaded part containing
   everything until the "evtchn->enabled = false" and only then
   kick the thread doing the rest, including the spin_lock().

Juergen