netdev - Re: [PATCH net] r8169: fix operation under forced interrupt threading

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Message-ID: <b976846d-f40e-961f-6a3e-920fd5bf1add@gmail.com>
Date:   Thu, 29 Oct 2020 11:19:14 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Serge Belyshev <belyshev@...ni.sinp.msu.ru>,
        David Miller <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] r8169: fix operation under forced interrupt threading

On 29.10.2020 10:42, Thomas Gleixner wrote:
> On Thu, Oct 29 2020 at 09:42, Heiner Kallweit wrote:
>> On 29.10.2020 00:29, Jakub Kicinski wrote:
>>> Other handles may take spin_locks, which will sleep on RT.
>>>
>>> I guess we may need to switch away from the _irqoff() variant for
>>> drivers with IRQF_SHARED after all :(
>>>
>> Right. Unfortunately that's a large number of drivers,
>> e.g. pci_request_irq() sets IRQF_SHARED in general.
> 
> IRQF_SHARED is not the problem. It only becomes a problem when the
> interrupt is actually shared which is only the case with the legacy PCI
> interrupt. MSI[X] is not affected at all.
> 
Correct, just that the legacy PCI interrupt scenario doesn't affect old
systems/devices only. Users may run the system with nomsi for
whatever reason and we need to be prepared.

We could add handling for (pcidev->msi_enabled || pcidev->msix_enabled),
but this would look somewhat hacky to me.

>> But at least for now there doesn't seem to be a better way to deal
>> with the challenges imposed by forced threading and shared irqs.
> 
> We still can do the static key trick, though it's admittedly hacky.
> 
> Thanks,
> 
>         tglx
> 
>