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

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Message-ID: <a37b2cdf-97c4-8d13-2a49-d4f8c0b43f04@gmail.com>
Date:   Thu, 29 Oct 2020 09:42:41 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Serge Belyshev <belyshev@...ni.sinp.msu.ru>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 00:29, Jakub Kicinski wrote:
> On Wed, 28 Oct 2020 13:17:58 +0100 Heiner Kallweit wrote:
>> On 28.10.2020 12:43, Serge Belyshev wrote:
>>>> For several network drivers it was reported that using
>>>> __napi_schedule_irqoff() is unsafe with forced threading. One way to
>>>> fix this is switching back to __napi_schedule, but then we lose the
>>>> benefit of the irqoff version in general. As stated by Eric it doesn't
>>>> make sense to make the minimal hard irq handlers in drivers using NAPI
>>>> a thread. Therefore ensure that the hard irq handler is never
>>>> thread-ified.
>>>>
>>>> Fixes: 9a899a35b0d6 ("r8169: switch to napi_schedule_irqoff")
>>>> Link: https://lkml.org/lkml/2020/10/18/19
>>>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
>>>> ---
>>>>  drivers/net/ethernet/realtek/r8169_main.c | 8 ++++----
>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
>>>> index 7d366b036..3b6ddc706 100644
>>>> --- a/drivers/net/ethernet/realtek/r8169_main.c
>>>> +++ b/drivers/net/ethernet/realtek/r8169_main.c  
>>> ...
>>>
>>> Hi!  This patch actually breaks r8169 with threadirqs on an old box
>>> where it was working before:
>>>
>>> [    0.000000] DMI: Gigabyte Technology Co., Ltd. GA-MA790FX-DQ6/GA-MA790FX-DQ6, BIOS F7g 07/19/2010
>>> ...
>>> [    1.072676] r8169 0000:02:00.0 eth0: RTL8168b/8111b, 00:1a:4d:5d:6b:c3, XID 380, IRQ 18
>>> ...
>>> [    8.850099] genirq: Flags mismatch irq 18. 00010080 (eth0) vs. 00002080 (ahci[0000:05:00.0])
>>>
>>> (error is reported to userspace, interface failed to bring up).
>>> Reverting the patch fixes the problem.
>>>   
>> Thanks for the report. On this old chip version MSI is unreliable,
>> therefore r8169 falls back to a PCI legacy interrupt. On your system
>> this PCI legacy interrupt seems to be shared between network and
>> disk. Then the IRQ core tries to threadify the disk interrupt
>> (setting IRQF_ONESHOT), whilst the network interrupt doesn't have
>> this flag set. This results in the flag mismatch error.
>>
>> Maybe, if one source of a shared interrupt doesn't allow forced
>> threading, this should be applied to the other sources too.
>> But this would require a change in the IRQ core, therefore
>> +Thomas to get his opinion on the issue.
> 
> 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.
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.