From: Long Li <longli@xxxxxxxxxxxxx> 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 = 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 = debugfs_create_dir("az_blob", NULL);
> > > > > + if (!IS_ERR_OR_NULL(az_blob_debugfs_root)) {
> > > > > + d = 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 = 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_dev
> > > > 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 guarantee
> > > 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 middle 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
>
> 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).
>
> However, this works fine for "rmmod" that causes device to be removed. Before file is opened the refcount on the module
> is increased so it can't be removed when file is being opened. The scenario you described can't happen.
>
> 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
>
> This is a very rare. I agree things happen that we should make sure the driver can handle this. I'll update the driver.
>
> Long
>
> >
> > >
> > > >
> > > > > + INIT_LIST_HEAD(&az_blob_dev.file_list);
> > > > > + init_waitqueue_head(&az_blob_dev.file_wait);
> > > > > +
> > > > > + az_blob_dev.removing = false;
> > > > > +
> > > > > + az_blob_dev.device = device;
> > > > > + device->channel->rqstor_size = device_queue_depth;
> > > > > +
> > > > > + ret = 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 = 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 = 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 = hv_get_drvdata(dev);
> > > > > + unsigned long flags;
> > > > > +
> > > > > + spin_lock_irqsave(&device->file_lock, flags);
> > > > > + device->removing = 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 = {
> > > > > + .name = KBUILD_MODNAME,
> > > > > + .id_table = id_table,
> > > > > + .probe = az_blob_probe,
> > > > > + .remove = az_blob_remove,
> > > > > + .driver = {
> > > > > + .probe_type = PROBE_PREFER_ASYNCHRONOUS,
> > > > > + },
> > > > > +};
> > > > > +
> > > > > +static int __init az_blob_drv_init(void) {
> > > > > + int ret;
> > > > > +
> > > > > + ret = vmbus_driver_register(&az_blob_drv);
> > > > > + return ret;
> > > > > +}
> > > > > +
> > > > > +static void __exit az_blob_drv_exit(void) {
> > > > > + vmbus_driver_unregister(&az_blob_drv);
> > > > > +}
