linux-kernel - Re: [PATCH RFC v5 1/5] pci: report surprise removal event

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Message-ID: <20250714211351.GA2418892@bhelgaas>
Date: Mon, 14 Jul 2025 16:13:51 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
	Keith Busch <kbusch@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Parav Pandit <parav@...dia.com>, virtualization@...ts.linux.dev,
	stefanha@...hat.com, alok.a.tiwari@...cle.com,
	linux-pci@...r.kernel.org
Subject: Re: [PATCH RFC v5 1/5] pci: report surprise removal event

On Mon, Jul 14, 2025 at 02:26:19AM -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 09, 2025 at 06:38:20PM -0500, Bjorn Helgaas wrote:
> > On Wed, Jul 09, 2025 at 04:55:26PM -0400, Michael S. Tsirkin wrote:
> > > At the moment, in case of a surprise removal, the regular remove
> > > callback is invoked, exclusively.  This works well, because mostly, the
> > > cleanup would be the same.
> > > 
> > > However, there's a race: imagine device removal was initiated by a user
> > > action, such as driver unbind, and it in turn initiated some cleanup and
> > > is now waiting for an interrupt from the device. If the device is now
> > > surprise-removed, that never arrives and the remove callback hangs
> > > forever.
> > > 
> > > For example, this was reported for virtio-blk:
> > > 
> > > 	1. the graceful removal is ongoing in the remove() callback, where disk
> > > 	   deletion del_gendisk() is ongoing, which waits for the requests +to
> > > 	   complete,
> > > 
> > > 	2. Now few requests are yet to complete, and surprise removal started.
> > > 
> > > 	At this point, virtio block driver will not get notified by the driver
> > > 	core layer, because it is likely serializing remove() happening by
> > > 	+user/driver unload and PCI hotplug driver-initiated device removal.  So
> > > 	vblk driver doesn't know that device is removed, block layer is waiting
> > > 	for requests completions to arrive which it never gets.  So
> > > 	del_gendisk() gets stuck.
> > > 
> > > Drivers can artificially add timeouts to handle that, but it can be
> > > flaky.
> > > 
> > > Instead, let's add a way for the driver to be notified about the
> > > disconnect. It can then do any necessary cleanup, knowing that the
> > > device is inactive.
> > 
> > This relies on somebody (typically pciehp, I guess) calling
> > pci_dev_set_disconnected() when a surprise remove happens.
> > 
> > Do you think it would be practical for the driver's .remove() method
> > to recognize that the device may stop responding at any point, even if
> > no hotplug driver is present to call pci_dev_set_disconnected()?
> > 
> > Waiting forever for an interrupt seems kind of vulnerable in general.
> > Maybe "artificially adding timeouts" is alluding to *not* waiting
> > forever for interrupts?  That doesn't seem artificial to me because
> > it's just a fact of life that devices can disappear at arbitrary
> > times.
> > 
> > It seems a little fragile to me to depend on some other part of the
> > system to notice the surprise removal and tell you about it or
> > schedule your work function.  I think it would be more robust for the
> > driver to check directly, i.e., assume writes to the device may be
> > lost, check for PCI_POSSIBLE_ERROR() after reads from the device, and
> > never wait for an interrupt without a timeout.
> 
> virtio is ... kind of special, in that users already take it for
> granted that having a device as long as they want to respond
> still does not lead to errors and data loss.
> 
> Makes it a bit harder as our timeout would have to
> check for presence and retry, we can't just fail as a
> normal hardware device does.

Sorry, I don't know enough about virtio to follow what you said about 
"having a device as long as they want to respond".

We started with a graceful remove.  That must mean the user no longer
needs the device.

> And there's the overhead thing - poking at the device a lot
> puts a high load on the host.

Checking for PCI_POSSIBLE_ERROR() doesn't touch the device.  If you
did a config read already, and the result happened to be ~0, *then* we
have the problem of figuring out whether the actual data from the
device was ~0, or if the read failed and the Root Complex synthesized
the ~0.  In many cases a driver knows that ~0 is not a possible
register value.  Otherwise it might have to read another register that
is known not to be ~0.

Bjorn