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

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Message-ID: <1831964f-185e-31bb-2446-778f2c18d71b@suse.com>
Date:   Mon, 8 Feb 2021 13:14:13 +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 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? And I think we want to keep
the lateeoi behavior in order to be able to control event storms.


Juergen

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