Re: [PATCH v2] pci: fix device presence detection for VFs

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 10, 2022 at 01:35:47PM -0600, Bjorn Helgaas wrote:
> Hi Wei,
> 
> I can't quite parse this.  Is the problem that you had some virtio I/O
> in progress, you wrote "0" to /sys/.../sriov_numvfs, and the virtio
> I/O operation hangs?

I think so. I also think that just attempting to
remove the module or to unbind the driver from it
will have the same effect.

> Is there any indication to the user, e.g., softlockup oops?
> 
> More questions below.
> 
> On Tue, Nov 08, 2022 at 04:52:16AM +0000, Wei Gong wrote:
> 
> > according to sriov's protocol specification vendor_id and
> > device_id field in all VFs return FFFFh when read
> > so when vf devs is in the pci_device_is_present,it will be
> > misjudged as surprise removeal
> > 
> > when io is issued on the vf, normally disable virtio_blk vf
> > devs,at this time the disable opration will hang. and virtio
> > blk dev io hang.
> >
> > task:bash            state:D stack:    0 pid: 1773 ppid:  1241 flags:0x00004002
> > Call Trace:
> >  <TASK>
> >  __schedule+0x2ee/0x900
> >  schedule+0x4f/0xc0
> >  blk_mq_freeze_queue_wait+0x69/0xa0
> >  ? wait_woken+0x80/0x80
> >  blk_mq_freeze_queue+0x1b/0x20
> >  blk_cleanup_queue+0x3d/0xd0
> >  virtblk_remove+0x3c/0xb0 [virtio_blk]
> >  virtio_dev_remove+0x4b/0x80
> >  device_release_driver_internal+0x103/0x1d0
> >  device_release_driver+0x12/0x20
> >  bus_remove_device+0xe1/0x150
> >  device_del+0x192/0x3f0
> >  device_unregister+0x1b/0x60
> >  unregister_virtio_device+0x18/0x30
> >  virtio_pci_remove+0x41/0x80
> >  pci_device_remove+0x3e/0xb0
> >  device_release_driver_internal+0x103/0x1d0
> >  device_release_driver+0x12/0x20
> >  pci_stop_bus_device+0x79/0xa0
> >  pci_stop_and_remove_bus_device+0x13/0x20
> >  pci_iov_remove_virtfn+0xc5/0x130
> >  ? pci_get_device+0x4a/0x60
> >  sriov_disable+0x33/0xf0
> >  pci_disable_sriov+0x26/0x30
> >  virtio_pci_sriov_configure+0x6f/0xa0
> >  sriov_numvfs_store+0x104/0x140
> >  dev_attr_store+0x17/0x30
> >  sysfs_kf_write+0x3e/0x50
> >  kernfs_fop_write_iter+0x138/0x1c0
> >  new_sync_write+0x117/0x1b0
> >  vfs_write+0x185/0x250
> >  ksys_write+0x67/0xe0
> >  __x64_sys_write+0x1a/0x20
> >  do_syscall_64+0x61/0xb0
> >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > RIP: 0033:0x7f21bd1f3ba4
> > RSP: 002b:00007ffd34a24188 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
> > RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007f21bd1f3ba4
> > RDX: 0000000000000002 RSI: 0000560305040800 RDI: 0000000000000001
> > RBP: 0000560305040800 R08: 000056030503fd50 R09: 0000000000000073
> > R10: 00000000ffffffff R11: 0000000000000202 R12: 0000000000000002
> > R13: 00007f21bd2de760 R14: 00007f21bd2da5e0 R15: 00007f21bd2d99e0
> > 
> > when virtio_blk is performing io, as long as there two stages of:
> > 1. dispatch io. queue_usage_counter++;
> > 2. io is completed after receiving the interrupt. queue_usage_counter--;
> > 
> > disable virtio_blk vfs:
> >   if(!pci_device_is_present(pci_dev))
> >     virtio_break_device(&vp_dev->vdev);
> > virtqueue for vf devs will be marked broken.
> > the interrupt notification io is end. Since it is judged that the
> > virtqueue has been marked as broken, the completed io will not be
> > performed.
> > So queue_usage_counter will not be cleared.
> > when the disk is removed at the same time, the queue will be frozen,
> > and you must wait for the queue_usage_counter to be cleared.
> > Therefore, it leads to the removeal of hang.
> 
> I want to follow along in the code, but I need some hints.
> 
> "queue_usage_counter" looks like it's supposed to be a symbol, but I
> can't find it.

I think it refers to q->q_usage_counter in blk core.

> Where (which function) is the I/O dispatched and queue_usage_counter
> incremented?  Where is queue_usage_counter decremented?
> 
> Prior to this change pci_device_is_present(VF) returned "false"
> (because the VF Vendor ID is 0xffff); after the change it will return
> "true" (because it will look at the PF Vendor ID instead).
> 
> Previously virtio_pci_remove() called virtio_break_device().  I guess
> that meant the virtio I/O operation will never be completed?
> 
> But if we don't call virtio_break_device(), the virtio I/O operation
> *will* be completed?
> 
> Bjorn

It's completed anyway - nothing special happened at the device
level - but driver does not detect it.

Calling virtio_break_device will mark all queues as broken, as
a result attempts to check whether operation completed
will return false.

This probably means we need to work on handling surprise removal
better in virtio blk - since it looks like actual suprise
removal will hang too. But that I think is a separate issue.

-- 
MST