From: Vineeth Vijayan <vneethv@linux.ibm.com>
To: Halil Pasic <pasic@linux.ibm.com>
Cc: Cornelia Huck <cohuck@redhat.com>,
Heiko Carstens <hca@linux.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>,
Christian Borntraeger <borntraeger@de.ibm.com>,
Pierre Morel <pmorel@linux.ibm.com>,
Michael Mueller <mimu@linux.ibm.com>,
linux-s390@vger.kernel.org,
virtualization@lists.linux-foundation.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, bfu@redhat.com,
Peter Oberparleiter <oberpar@linux.ibm.com>
Subject: Re: [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown
Date: Tue, 21 Sep 2021 20:25:09 +0200 [thread overview]
Message-ID: <f4dc7040554fd7e9c7067aab2213b3639cfc6987.camel@linux.ibm.com> (raw)
In-Reply-To: <20210921185222.246b15bb.pasic@linux.ibm.com>
On Tue, 2021-09-21 at 18:52 +0200, Halil Pasic wrote:
> > > > lock already,
> > >
> > > I believe we have a misunderstanding here. I believe that Vineeth
> > > is
> > > trying to tell us, that online_store_handle_offline() and
> > > online_store_handle_offline() are called under the a device lock
> > > of
> > > the ccw device. Right, Vineeth?
> > Yes. I wanted to bring-out both the scenario.The
> > set_offline/_online()
> > calls and the unconditional-remove call.
>
> I don't understand the paragraph above. I can't map the terms
> set_offline/_online() and unconditional-remove call to chunks of
> code.
> :(
online_store() function can be used to set_online/set_offline manually
from the sysfs entry.
And an unconditional-remove call, for CIO, starts with a CRW which
indicates there is a subchannel_event which needs to be taken care.
This sch_event() (in device.c) then try to find the reason for this CRW
and act accordingly. This would lead to device_del and end up calling
the remove function of the driver.
> > For the set_online The virtio_ccw_online() also invoked with
> > ccwlock
> > held. (ref: ccw_device_set_online)
>
> I don't think virtio_ccw_online() is invoked with the ccwlock held. I
> think we call virtio_ccw_online() in this line:
> https://elixir.bootlin.com/linux/v5.15-rc2/source/drivers/s390/cio/device.c#L394
> and we have released the cdev->ccwlock literally 2 lines above.
My bad. I overlooked it!
>
>
> > > Conny, I believe, by online/offline callbacks, you mean
> > > virtio_ccw_online() and virtio_ccw_offline(), right?
> > >
> > > But the thing is that virtio_ccw_online() may get called (and is
> > > typically called, AFAICT) with no locks held via:
> > > virtio_ccw_probe() --> async_schedule(virtio_ccw_auto_online,
> > > cdev)
> > > -*-> virtio_ccw_auto_online(cdev) --> ccw_device_set_online(cdev)
> > > -->
> > > virtio_ccw_online()
> > >
> > > Furthermore after a closer look, I believe because we don't take
> > > a reference to the cdev in probe, we may get
> > > virtio_ccw_auto_online()
> > > called with an invalid pointer (the pointer is guaranteed to be
> > > valid
> > > in probe, but because of async we have no guarantee that it will
> > > be
> > > called in the context of probe).
> > >
> > > Shouldn't we take a reference to the cdev in probe?
> > We just had a quick look at the virtio_ccw_probe() function.
> > Did you mean to have a get_device() during the probe() and
> > put_device()
> > just after the virtio_ccw_auto_online() ?
>
> Yes, that would ensure that cdev pointer is still valid when
> virtio_ccw_auto_online() is executed, and that things are cleaned up
> properly, I guess. But I'm not 100% sure about all the interactions.
> AFAIR ccw_device_set_online(cdev) would bail out if !drv. But then
> we have the case where we already assigned it to a new driver (e.g.
> vfio for dasd).
>
> BTW I believe if we have a problem here, the dasd driver has the same
> problem as well. The code looks very, very similar.
You are right about that. I am trying to recreate that issue with DASD
now. And working on the patch as well.
>
> And shouldn't this auto-online be common CIO functionality? What is
> the
> reason the char devices don't seem to have it?
I am not sure about that. I dont understand why it should be a CIO
functionality.
>
> Regards,
> Halil
next prev parent reply other threads:[~2021-09-21 18:25 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-15 21:57 [PATCH 1/1] virtio/s390: fix vritio-ccw device teardown Halil Pasic
2021-09-15 22:00 ` Halil Pasic
2021-09-16 8:59 ` Cornelia Huck
2021-09-16 13:18 ` Halil Pasic
2021-09-17 8:40 ` Cornelia Huck
2021-09-19 22:39 ` Halil Pasic
2021-09-20 7:41 ` Vineeth Vijayan
2021-09-20 10:07 ` Cornelia Huck
2021-09-21 3:25 ` Halil Pasic
2021-09-21 12:09 ` Cornelia Huck
2021-09-21 13:31 ` Vineeth Vijayan
2021-09-21 16:52 ` Halil Pasic
2021-09-21 18:25 ` Vineeth Vijayan [this message]
2021-09-20 10:30 ` Cornelia Huck
2021-09-20 13:27 ` Halil Pasic
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