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

[Bjorn Helgaas <helgaas@kernel.org>]

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?

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.

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