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To:     Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        netdev@vger.kernel.org, linux-scsi@vger.kernel.org
Cc:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        stable@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
        Jens Axboe <axboe@kernel.dk>, Wei Liu <wei.liu@kernel.org>,
        Paul Durrant <paul@xen.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
References: <20210206104932.29064-1-jgross@suse.com>
 <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>
 <eeb62129-d9fc-2155-0e0f-aff1fbb33fbc@suse.com>
 <fcf3181b-3efc-55f5-687c-324937b543e6@xen.org>
 <7aaeeb3d-1e1b-6166-84e9-481153811b62@suse.com>
 <6f547bb5-777a-6fc2-eba2-cccb4adfca87@xen.org>
 <0d623c98-a714-1639-cc53-f58ba3f08212@suse.com>
 <28399fd1-9fe8-f31a-6ee8-e78de567155b@xen.org>
 <1831964f-185e-31bb-2446-778f2c18d71b@suse.com>
 <e8c46e36-cf9e-fb30-21b5-fa662834a01a@xen.org>
 <199b76fd-630b-a0c6-926b-3e662103ec42@suse.com>
 <063eff75-56a5-1af7-f684-a2ed4b13c9a7@xen.org>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
Message-ID: <4279cab9-9b36-e83d-bd7a-ff7cd2832054@suse.com>
Date:   Mon, 8 Feb 2021 14:58:11 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
To: Julien Grall <julien@xen.org>, xen-devel@lists.xenproject.org,
 linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
 netdev@vger.kernel.org, linux-scsi@vger.kernel.org
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
 Stefano Stabellini <sstabellini@kernel.org>, stable@vger.kernel.org,
 Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
 =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= <roger.pau@citrix.com>,
 Jens Axboe <axboe@kernel.dk>, Wei Liu <wei.liu@kernel.org>,
 Paul Durrant <paul@xen.org>, "David S. Miller" <davem@davemloft.net>,
 Jakub Kicinski <kuba@kernel.org>
Message-ID: <4279cab9-9b36-e83d-bd7a-ff7cd2832054@suse.com>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
References: <20210206104932.29064-1-jgross@suse.com>
 <bd63694e-ac0c-7954-ec00-edad05f8da1c@xen.org>
 <eeb62129-d9fc-2155-0e0f-aff1fbb33fbc@suse.com>
 <fcf3181b-3efc-55f5-687c-324937b543e6@xen.org>
 <7aaeeb3d-1e1b-6166-84e9-481153811b62@suse.com>
 <6f547bb5-777a-6fc2-eba2-cccb4adfca87@xen.org>
 <0d623c98-a714-1639-cc53-f58ba3f08212@suse.com>
 <28399fd1-9fe8-f31a-6ee8-e78de567155b@xen.org>
 <1831964f-185e-31bb-2446-778f2c18d71b@suse.com>
 <e8c46e36-cf9e-fb30-21b5-fa662834a01a@xen.org>
 <199b76fd-630b-a0c6-926b-3e662103ec42@suse.com>
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On 08.02.21 14:09, Julien Grall wrote:
> Hi Juergen,
>=20
> On 08/02/2021 12:31, J=C3=BCrgen Gro=C3=9F wrote:
>> On 08.02.21 13:16, Julien Grall wrote:
>>>
>>>
>>> On 08/02/2021 12:14, J=C3=BCrgen Gro=C3=9F wrote:
>>>> On 08.02.21 11:40, Julien Grall wrote:
>>>>> Hi Juergen,
>>>>>
>>>>> On 08/02/2021 10:22, J=C3=BCrgen Gro=C3=9F wrote:
>>>>>> On 08.02.21 10:54, Julien Grall wrote:
>>>>>>> ... I don't really see how the difference matter here. The idea=20
>>>>>>> is to re-use what's already existing rather than trying to=20
>>>>>>> re-invent the wheel with an extra lock (or whatever we can come u=
p).
>>>>>>
>>>>>> The difference is that the race is occurring _before_ any IRQ is
>>>>>> involved. So I don't see how modification of IRQ handling would he=
lp.
>>>>>
>>>>> Roughly our current IRQ handling flow (handle_eoi_irq()) looks like=
:
>>>>>
>>>>> if ( irq in progress )
>>>>> {
>>>>> =C2=A0=C2=A0 set IRQS_PENDING
>>>>> =C2=A0=C2=A0 return;
>>>>> }
>>>>>
>>>>> do
>>>>> {
>>>>> =C2=A0=C2=A0 clear IRQS_PENDING
>>>>> =C2=A0=C2=A0 handle_irq()
>>>>> } while (IRQS_PENDING is set)
>>>>>
>>>>> IRQ handling flow like handle_fasteoi_irq() looks like:
>>>>>
>>>>> if ( irq in progress )
>>>>> =C2=A0=C2=A0 return;
>>>>>
>>>>> handle_irq()
>>>>>
>>>>> The latter flow would catch "spurious" interrupt and ignore them.=20
>>>>> So it would handle nicely the race when changing the event affinity=
=2E
>>>>
>>>> Sure? Isn't "irq in progress" being reset way before our "lateeoi" i=
s
>>>> issued, thus having the same problem again?=20
>>>
>>> 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 anothe=
r
>> IRQ?
> It is reset after the handler has been called. See handle_irq_event().

Right. And for us this is too early, as we want the next IRQ being
handled only after we have called xen_irq_lateeoi().

>=20
>> 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 w=
e
>> are experiencing now.
>=20
> I am a bit confused what you mean by "lateeoi" handling is becoming=20
> active. Can you clarify?

See above: the next call of the handler should be allowed only after
xen_irq_lateeoi() for the IRQ has been called.

If the handler is being called earlier we have the race resulting
in the WARN() splats.

> Note that are are other IRQ flows existing. We should have a look at=20
> them before trying to fix thing ourself.

Fine with me, but it either needs to fit all use cases (interdomain,
IPI, real interrupts) or we need to have a per-type IRQ flow.

I think we should fix the issue locally first, then we can start to do
a thorough rework planning. Its not as if the needed changes with the
current flow would be so huge, and I'd really like to have a solution
rather sooner than later. Changing the IRQ flow might have other side
effects which need to be excluded by thorough testing.

> Although, the other issue I can see so far is handle_irq_for_port() wil=
l=20
> update info->{eoi_cpu, irq_epoch, eoi_time} without any locking. But it=
=20
> is not clear this is what you mean by "becoming active".

As long as a single event can't be handled on multiple cpus at the same
time, there is no locking needed.


Juergen

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