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From:   Michael Kelley <mikelley@microsoft.com>
To:     Long Li <longli@microsoft.com>,
        "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
CC:     Jonathan Corbet <corbet@lwn.net>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Hans de Goede <hdegoede@redhat.com>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        Maximilian Luz <luzmaximilian@gmail.com>,
        Mike Rapoport <rppt@kernel.org>,
        Ben Widawsky <ben.widawsky@intel.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        Andra Paraschiv <andraprs@amazon.com>,
        Siddharth Gupta <sidgup@codeaurora.org>,
        Hannes Reinecke <hare@suse.de>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
Subject: RE: [Patch v2 2/3] Drivers: hv: add Azure Blob driver
Thread-Topic: [Patch v2 2/3] Drivers: hv: add Azure Blob driver
Thread-Index: AQHXalTRwSwb8zgU/UyKv3LljLGzrasphsOQgAQvmICAAJcUUIACGm+AgAmQCRA=
Date:   Fri, 9 Jul 2021 02:09:12 +0000
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From: Long Li <longli@microsoft.com> Sent: Friday, July 2, 2021 4:59 PM
> > PM
> >
> > [snip]
> >
> > > > > +static void az_blob_remove_device(struct az_blob_device *dev) {
> > > > > +	wait_event(dev->file_wait, list_empty(&dev->file_list));
> > > > > +	misc_deregister(&az_blob_misc_device);
> > > > > +#ifdef CONFIG_DEBUG_FS
> > > > > +	debugfs_remove_recursive(az_blob_debugfs_root);
> > > > > +#endif
> > > > > +	/* At this point, we won't get any requests from user-mode */ }
> > > > > +
> > > > > +static int az_blob_create_device(struct az_blob_device *dev) {
> > > > > +	int rc;
> > > > > +	struct dentry *d;
> > > > > +
> > > > > +	rc =3D misc_register(&az_blob_misc_device);
> > > > > +	if (rc) {
> > > > > +		az_blob_err("misc_register failed rc %d\n", rc);
> > > > > +		return rc;
> > > > > +	}
> > > > > +
> > > > > +#ifdef CONFIG_DEBUG_FS
> > > > > +	az_blob_debugfs_root =3D debugfs_create_dir("az_blob", NULL);
> > > > > +	if (!IS_ERR_OR_NULL(az_blob_debugfs_root)) {
> > > > > +		d =3D debugfs_create_file("pending_requests", 0400,
> > > > > +			az_blob_debugfs_root, NULL,
> > > > > +			&az_blob_debugfs_fops);
> > > > > +		if (IS_ERR_OR_NULL(d)) {
> > > > > +			az_blob_warn("failed to create debugfs file\n");
> > > > > +			debugfs_remove_recursive(az_blob_debugfs_root);
> > > > > +			az_blob_debugfs_root =3D NULL;
> > > > > +		}
> > > > > +	} else
> > > > > +		az_blob_warn("failed to create debugfs root\n"); #endif
> > > > > +
> > > > > +	return 0;
> > > > > +}
> > > > > +
> > > > > +static int az_blob_connect_to_vsp(struct hv_device *device, u32
> > > > > +ring_size) {
> > > > > +	int ret;
> > > > > +
> > > > > +	spin_lock_init(&az_blob_dev.file_lock);
> > > >
> > > > I'd argue that the spin lock should not be re-initialized here.
> > > > Here's the sequence where things go wrong:
> > > >
> > > > 1) The driver is unbound, so az_blob_remove() is called.
> > > > 2) az_blob_remove() sets the "removing" flag to true, and calls
> > > > az_blob_remove_device().
> > > > 3) az_blob_remove_device() waits for the file_list to become empty.
> > > > 4) After the file_list becomes empty, but before misc_deregister()
> > > > is called, a separate thread opens the device again.
> > > > 5) In the separate thread, az_blob_fop_open() obtains the file_lock=
 spin
> > lock.
> > > > 6) Before az_blob_fop_open() releases the spin lock,
> > > > az_blob_remove_device() completes, along with az_blob_remove().
> > > > 7) Then the device gets rebound, and az_blob_connect_to_vsp() gets
> > > > called, all while az_blob_fop_open() still holds the spin lock.  So
> > > > the spin lock get re- initialized while it is held.
> > > >
> > > > This is admittedly a far-fetched scenario, but stranger things have
> > > > happened. :-)  The issue is that you are counting on the az_blob_de=
v
> > > > structure to persist and have a valid file_lock, even while the
> > > > device is unbound.  So any initialization should only happen in
> > az_blob_drv_init().
> > >
> > > I'm not sure if az_blob_probe() and az_blob_remove() can be called at
> > > the same time, as az_blob_remove_vmbus() is called the last in
> > az_blob_remove().
> > > Is it possible for vmbus asking the driver to probe a new channel
> > > before the old channel is closed? I expect the vmbus provide guarante=
e
> > > that those calls are made in sequence.
> >
> > In my scenario above, az_blob_remove_vmbus() and az_blob_remove() run
> > to completion in Step #6, all while some other thread is still in the m=
iddle of
> > an
> > open() call and holding the file_lock spin lock.  Then in Step #7
> > az_blob_probe() runs.  So az_blob_remove() and az_blob_probe() execute
> > sequentially, not at the same time.
> >
> > Michael
>=20
> I think it's a valid scenario.  The return value of devtmpfs_delete_node(=
) is not checked in misc_deregister(). It decreases
> the refcount on inodes but it's not guaranteed that someone else is still=
 using it (in the middle of opening a file).
>=20
> However, this works fine for "rmmod" that causes device to be removed. Be=
fore file is opened the refcount on the module
> is increased so it can't be removed when file is being opened. The scenar=
io you described can't happen.
>=20
> But during VMBUS rescind, it can happen. It's possible that the driver is=
 using the spinlock that has been re-initialized, when
> the next VMBUS offer on the same channel comes before all the attempting =
open file calls exit.

I was thinking about the rescind scenario.  vmbus_onoffer_rescind()
will run on the global workqueue.  If it eventually calls az_blob_remove()
and then az_blob_remove_device(), it will wait until the file_list is
empty, which essentially means waiting until user space processes
decide to close the instances they have open.  This seems like a
problem that could block the global workqueue for a long time and
thereby hang the kernel.   Is my reasoning valid?  If so, I haven't
thought about what the solution might be.  It seems like we do need
to wait until any in-progress requests to Hyper-V are complete because
Hyper-V has references to guest physical memory.  But waiting for
all open instances to be closed seems to be problematic.

Michael

>=20
> This is a very rare. I agree things happen that we should make sure the d=
river can handle this. I'll update the driver.
>=20
> Long
>=20
> >
> > >
> > > >
> > > > > +	INIT_LIST_HEAD(&az_blob_dev.file_list);
> > > > > +	init_waitqueue_head(&az_blob_dev.file_wait);
> > > > > +
> > > > > +	az_blob_dev.removing =3D false;
> > > > > +
> > > > > +	az_blob_dev.device =3D device;
> > > > > +	device->channel->rqstor_size =3D device_queue_depth;
> > > > > +
> > > > > +	ret =3D vmbus_open(device->channel, ring_size, ring_size, NULL,=
 0,
> > > > > +			az_blob_on_channel_callback, device->channel);
> > > > > +
> > > > > +	if (ret) {
> > > > > +		az_blob_err("failed to connect to VSP ret %d\n", ret);
> > > > > +		return ret;
> > > > > +	}
> > > > > +
> > > > > +	hv_set_drvdata(device, &az_blob_dev);
> > > > > +
> > > > > +	return ret;
> > > > > +}
> > > > > +
> > > > > +static void az_blob_remove_vmbus(struct hv_device *device) {
> > > > > +	/* At this point, no VSC/VSP traffic is possible over vmbus */
> > > > > +	hv_set_drvdata(device, NULL);
> > > > > +	vmbus_close(device->channel);
> > > > > +}
> > > > > +
> > > > > +static int az_blob_probe(struct hv_device *device,
> > > > > +			const struct hv_vmbus_device_id *dev_id) {
> > > > > +	int rc;
> > > > > +
> > > > > +	az_blob_dbg("probing device\n");
> > > > > +
> > > > > +	rc =3D az_blob_connect_to_vsp(device, az_blob_ringbuffer_size);
> > > > > +	if (rc) {
> > > > > +		az_blob_err("error connecting to VSP rc %d\n", rc);
> > > > > +		return rc;
> > > > > +	}
> > > > > +
> > > > > +	// create user-mode client library facing device
> > > > > +	rc =3D az_blob_create_device(&az_blob_dev);
> > > > > +	if (rc) {
> > > > > +		az_blob_remove_vmbus(device);
> > > > > +		return rc;
> > > > > +	}
> > > > > +
> > > > > +	az_blob_dbg("successfully probed device\n");
> > > > > +	return 0;
> > > > > +}
> > > > > +
> > > > > +static int az_blob_remove(struct hv_device *dev) {
> > > > > +	struct az_blob_device *device =3D hv_get_drvdata(dev);
> > > > > +	unsigned long flags;
> > > > > +
> > > > > +	spin_lock_irqsave(&device->file_lock, flags);
> > > > > +	device->removing =3D true;
> > > > > +	spin_unlock_irqrestore(&device->file_lock, flags);
> > > > > +
> > > > > +	az_blob_remove_device(device);
> > > > > +	az_blob_remove_vmbus(dev);
> > > > > +	return 0;
> > > > > +}
> > > > > +
> > > > > +static struct hv_driver az_blob_drv =3D {
> > > > > +	.name =3D KBUILD_MODNAME,
> > > > > +	.id_table =3D id_table,
> > > > > +	.probe =3D az_blob_probe,
> > > > > +	.remove =3D az_blob_remove,
> > > > > +	.driver =3D {
> > > > > +		.probe_type =3D PROBE_PREFER_ASYNCHRONOUS,
> > > > > +	},
> > > > > +};
> > > > > +
> > > > > +static int __init az_blob_drv_init(void) {
> > > > > +	int ret;
> > > > > +
> > > > > +	ret =3D vmbus_driver_register(&az_blob_drv);
> > > > > +	return ret;
> > > > > +}
> > > > > +
> > > > > +static void __exit az_blob_drv_exit(void) {
> > > > > +	vmbus_driver_unregister(&az_blob_drv);
> > > > > +}