netdev - Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

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Date:   Mon, 8 Feb 2021 13:31:36 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Julien Grall <julien@....org>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        netdev@...r.kernel.org, linux-scsi@...r.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Roger Pau Monné <roger.pau@...rix.com>,
        Jens Axboe <axboe@...nel.dk>, Wei Liu <wei.liu@...nel.org>,
        Paul Durrant <paul@....org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids

On 08.02.21 13:16, Julien Grall wrote:
> 
> 
> On 08/02/2021 12:14, Jürgen Groß wrote:
>> On 08.02.21 11:40, Julien Grall wrote:
>>> Hi Juergen,
>>>
>>> On 08/02/2021 10:22, Jürgen Groß wrote:
>>>> On 08.02.21 10:54, Julien Grall wrote:
>>>>> ... I don't really see how the difference matter here. The idea is 
>>>>> to re-use what's already existing rather than trying to re-invent 
>>>>> the wheel with an extra lock (or whatever we can come up).
>>>>
>>>> The difference is that the race is occurring _before_ any IRQ is
>>>> involved. So I don't see how modification of IRQ handling would help.
>>>
>>> Roughly our current IRQ handling flow (handle_eoi_irq()) looks like:
>>>
>>> if ( irq in progress )
>>> {
>>>    set IRQS_PENDING
>>>    return;
>>> }
>>>
>>> do
>>> {
>>>    clear IRQS_PENDING
>>>    handle_irq()
>>> } while (IRQS_PENDING is set)
>>>
>>> IRQ handling flow like handle_fasteoi_irq() looks like:
>>>
>>> if ( irq in progress )
>>>    return;
>>>
>>> handle_irq()
>>>
>>> The latter flow would catch "spurious" interrupt and ignore them. So 
>>> it would handle nicely the race when changing the event affinity.
>>
>> Sure? Isn't "irq in progress" being reset way before our "lateeoi" is
>> issued, thus having the same problem again? 
> 
> Sorry I can't parse this.

handle_fasteoi_irq() will do nothing "if ( irq in progress )". When is
this condition being reset again in order to be able to process another
IRQ? I believe this will be the case before our "lateeoi" handling is
becoming active (more precise: when our IRQ handler is returning to
handle_fasteoi_irq()), resulting in the possibility of the same race we
are experiencing now.


Juergen

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