From: Julien Grall <julien@xen.org>
To: "Jürgen Groß" <jgross@suse.com>,
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>,
"Roger Pau Monné" <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>
Subject: Re: [PATCH 0/7] xen/events: bug fixes and some diagnostic aids
Date: Mon, 8 Feb 2021 09:54:13 +0000 [thread overview]
Message-ID: <6f547bb5-777a-6fc2-eba2-cccb4adfca87@xen.org> (raw)
In-Reply-To: <7aaeeb3d-1e1b-6166-84e9-481153811b62@suse.com>
On 08/02/2021 09:41, Jürgen Groß wrote:
> On 08.02.21 10:11, Julien Grall wrote:
>> Hi Juergen,
>>
>> On 07/02/2021 12:58, Jürgen Groß wrote:
>>> On 06.02.21 19:46, Julien Grall wrote:
>>>> Hi Juergen,
>>>>
>>>> On 06/02/2021 10:49, Juergen Gross wrote:
>>>>> The first three patches are fixes for XSA-332. The avoid WARN splats
>>>>> and a performance issue with interdomain events.
>>>>
>>>> Thanks for helping to figure out the problem. Unfortunately, I still
>>>> see reliably the WARN splat with the latest Linux master
>>>> (1e0d27fce010) + your first 3 patches.
>>>>
>>>> I am using Xen 4.11 (1c7d984645f9) and dom0 is forced to use the 2L
>>>> events ABI.
>>>>
>>>> After some debugging, I think I have an idea what's went wrong. The
>>>> problem happens when the event is initially bound from vCPU0 to a
>>>> different vCPU.
>>>>
>>>> From the comment in xen_rebind_evtchn_to_cpu(), we are masking the
>>>> event to prevent it being delivered on an unexpected vCPU. However,
>>>> I believe the following can happen:
>>>>
>>>> vCPU0 | vCPU1
>>>> |
>>>> | Call xen_rebind_evtchn_to_cpu()
>>>> receive event X |
>>>> | mask event X
>>>> | bind to vCPU1
>>>> <vCPU descheduled> | unmask event X
>>>> |
>>>> | receive event X
>>>> |
>>>> | handle_edge_irq(X)
>>>> handle_edge_irq(X) | -> handle_irq_event()
>>>> | -> set IRQD_IN_PROGRESS
>>>> -> set IRQS_PENDING |
>>>> | -> evtchn_interrupt()
>>>> | -> clear IRQD_IN_PROGRESS
>>>> | -> IRQS_PENDING is set
>>>> | -> handle_irq_event()
>>>> | -> evtchn_interrupt()
>>>> | -> WARN()
>>>> |
>>>>
>>>> All the lateeoi handlers expect a ONESHOT semantic and
>>>> evtchn_interrupt() is doesn't tolerate any deviation.
>>>>
>>>> I think the problem was introduced by 7f874a0447a9 ("xen/events: fix
>>>> lateeoi irq acknowledgment") because the interrupt was disabled
>>>> previously. Therefore we wouldn't do another iteration in
>>>> handle_edge_irq().
>>>
>>> I think you picked the wrong commit for blaming, as this is just
>>> the last patch of the three patches you were testing.
>>
>> I actually found the right commit for blaming but I copied the
>> information from the wrong shell :/. The bug was introduced by:
>>
>> c44b849cee8c ("xen/events: switch user event channels to lateeoi model")
>>
>>>
>>>> Aside the handlers, I think it may impact the defer EOI mitigation
>>>> because in theory if a 3rd vCPU is joining the party (let say vCPU A
>>>> migrate the event from vCPU B to vCPU C). So info->{eoi_cpu,
>>>> irq_epoch, eoi_time} could possibly get mangled?
>>>>
>>>> For a fix, we may want to consider to hold evtchn_rwlock with the
>>>> write permission. Although, I am not 100% sure this is going to
>>>> prevent everything.
>>>
>>> It will make things worse, as it would violate the locking hierarchy
>>> (xen_rebind_evtchn_to_cpu() is called with the IRQ-desc lock held).
>>
>> Ah, right.
>>
>>>
>>> On a first glance I think we'll need a 3rd masking state ("temporarily
>>> masked") in the second patch in order to avoid a race with lateeoi.
>>>
>>> In order to avoid the race you outlined above we need an "event is being
>>> handled" indicator checked via test_and_set() semantics in
>>> handle_irq_for_port() and reset only when calling clear_evtchn().
>>
>> It feels like we are trying to workaround the IRQ flow we are using
>> (i.e. handle_edge_irq()).
>
> I'm not really sure this is the main problem here. According to your
> analysis the main problem is occurring when handling the event, not when
> handling the IRQ: the event is being received on two vcpus.
I don't think we can easily divide the two because we rely on the IRQ
framework to handle the lifecycle of the event. So...
>
> Our problem isn't due to the IRQ still being pending, but due it being
> raised again, which should happen for a one shot IRQ the same way.
... 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).
Cheers,
--
Julien Grall
next prev parent reply other threads:[~2021-02-08 10:04 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-06 10:49 [PATCH 0/7] xen/events: bug fixes and some diagnostic aids Juergen Gross
2021-02-06 10:49 ` [PATCH 1/7] xen/events: reset affinity of 2-level event initially Juergen Gross
2021-02-06 11:20 ` Julien Grall
2021-02-06 12:09 ` Jürgen Groß
2021-02-06 12:19 ` Julien Grall
2021-02-06 10:49 ` [PATCH 2/7] xen/events: don't unmask an event channel when an eoi is pending Juergen Gross
2021-02-08 10:06 ` Jan Beulich
2021-02-08 10:21 ` Jürgen Groß
2021-02-08 10:15 ` Ross Lagerwall
2021-02-06 10:49 ` [PATCH 3/7] xen/events: fix lateeoi irq acknowledgment Juergen Gross
2021-02-06 10:49 ` [PATCH 4/7] xen/events: link interdomain events to associated xenbus device Juergen Gross
2021-02-08 23:26 ` Boris Ostrovsky
2021-02-09 13:55 ` Wei Liu
2021-02-06 10:49 ` [PATCH 5/7] xen/events: add per-xenbus device event statistics and settings Juergen Gross
2021-02-08 23:35 ` Boris Ostrovsky
2021-02-06 10:49 ` [PATCH 6/7] xen/evtch: use smp barriers for user event ring Juergen Gross
2021-02-08 9:38 ` Jan Beulich
2021-02-08 9:41 ` Jürgen Groß
2021-02-08 9:44 ` Andrew Cooper
2021-02-08 9:50 ` Jan Beulich
2021-02-08 10:23 ` Andrew Cooper
2021-02-08 10:25 ` Jürgen Groß
2021-02-08 10:31 ` Andrew Cooper
2021-02-08 10:36 ` Jan Beulich
2021-02-08 10:45 ` Andrew Cooper
2021-02-06 10:49 ` [PATCH 7/7] xen/evtchn: read producer index only once Juergen Gross
2021-02-08 9:48 ` Jan Beulich
2021-02-08 10:41 ` Jürgen Groß
2021-02-08 10:51 ` Jan Beulich
2021-02-08 10:59 ` Jürgen Groß
2021-02-08 11:50 ` Julien Grall
2021-02-08 11:54 ` Jan Beulich
2021-02-08 12:15 ` Jürgen Groß
2021-02-08 12:23 ` Jan Beulich
2021-02-08 12:26 ` Jürgen Groß
2021-02-08 11:40 ` Julien Grall
2021-02-08 11:48 ` Jürgen Groß
2021-02-08 12:03 ` Julien Grall
2021-02-06 18:46 ` [PATCH 0/7] xen/events: bug fixes and some diagnostic aids Julien Grall
2021-02-07 12:58 ` Jürgen Groß
2021-02-08 9:11 ` Julien Grall
2021-02-08 9:41 ` Jürgen Groß
2021-02-08 9:54 ` Julien Grall [this message]
2021-02-08 10:22 ` Jürgen Groß
2021-02-08 10:40 ` Julien Grall
2021-02-08 12:14 ` Jürgen Groß
2021-02-08 12:16 ` Julien Grall
2021-02-08 12:31 ` Jürgen Groß
2021-02-08 13:09 ` Julien Grall
2021-02-08 13:58 ` Jürgen Groß
2021-02-08 14:20 ` Julien Grall
2021-02-08 14:35 ` Julien Grall
2021-02-08 14:50 ` Jürgen Groß
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