Re: [PATCH 1/2] PCI/IOV: Add missing PCI rescan-remove locking when enabling/disabling SR-IOV

[Farhan Ali <alifm@linux.ibm.com>]

On 8/26/2025 1:52 AM, Niklas Schnelle wrote:
> Before disabling SR-IOV via config space accesses to the parent PF,
> sriov_disable() first removes the PCI devices representing the VFs.
>
> Since commit 9d16947b7583 ("PCI: Add global pci_lock_rescan_remove()")
> such removal operations are serialized against concurrent remove and
> rescan using the pci_rescan_remove_lock. No such locking was ever added
> in sriov_disable() however. In particular when commit 18f9e9d150fc
> ("PCI/IOV: Factor out sriov_add_vfs()") factored out the PCI device
> removal into sriov_del_vfs() there was still no locking around the
> pci_iov_remove_virtfn() calls.
>
> On s390 the lack of serialization in sriov_disable() may cause double
> remove and list corruption with the below (amended) trace being observed:
>
>   PSW:  0704c00180000000 0000000c914e4b38 (klist_put+56)
>   GPRS: 000003800313fb48 0000000000000000 0000000100000001 0000000000000001
>         00000000f9b520a8 0000000000000000 0000000000002fbd 00000000f4cc9480
>         0000000000000001 0000000000000000 0000000000000000 0000000180692828
>         00000000818e8000 000003800313fe2c 000003800313fb20 000003800313fad8
>   #0 [3800313fb20] device_del at c9158ad5c
>   #1 [3800313fb88] pci_remove_bus_device at c915105ba
>   #2 [3800313fbd0] pci_iov_remove_virtfn at c9152f198
>   #3 [3800313fc28] zpci_iov_remove_virtfn at c90fb67c0
>   #4 [3800313fc60] zpci_bus_remove_device at c90fb6104
>   #5 [3800313fca0] __zpci_event_availability at c90fb3dca
>   #6 [3800313fd08] chsc_process_sei_nt0 at c918fe4a2
>   #7 [3800313fd60] crw_collect_info at c91905822
>   #8 [3800313fe10] kthread at c90feb390
>   #9 [3800313fe68] __ret_from_fork at c90f6aa64
>   #10 [3800313fe98] ret_from_fork at c9194f3f2.
>
> This is because in addition to sriov_disable() removing the VFs, the
> platform also generates hot-unplug events for the VFs. This being
> the reverse operation to the hotplug events generated by sriov_enable()
> and handled via pdev->no_vf_scan. And while the event processing takes
> pci_rescan_remove_lock and checks whether the struct pci_dev still
> exists, the lack of synchronization makes this checking racy.
>
> Other races may also be possible of course though given that this lack
> of locking persisted so long obversable races seem very rare. Even on
> s390 the list corruption was only observed with certain devices since
> the platform events are only triggered by the config accesses that come
> after the removal, so as long as the removal finnished synchronously
> they would not race. Either way the locking is missing so fix this by
> adding it to the sriov_del_vfs() helper.
>
> Just lik PCI rescan-remove locking is also missing in sriov_add_vfs()
> including for the error case where pci_stop_ad_remove_bus_device() is
> called without the PCI rescan-remove lock being held. Even in the non
> error case adding new PCI devices and busses should be serialized via
> the PCI rescan-remove lock. Add the necessary locking.
>
> Cc: stable@vger.kernel.org
> Fixes: 18f9e9d150fc ("PCI/IOV: Factor out sriov_add_vfs()")
> Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>   drivers/pci/iov.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index ac4375954c9479b5f4a0e666b5215094fdaeefc2..77dee43b785838d215b58db2d22088e9346e0583 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -629,15 +629,18 @@ static int sriov_add_vfs(struct pci_dev *dev, u16 num_vfs)
>   	if (dev->no_vf_scan)
>   		return 0;
>   
> +	pci_lock_rescan_remove();
>   	for (i = 0; i < num_vfs; i++) {
>   		rc = pci_iov_add_virtfn(dev, i);

Should we move the lock/unlock to pci_iov_add_virtfn? As that's where 
the device is added to the bus? Similarly move the locking/unlocking to 
pci_iov_remove_virtfn?

Thanks
Farhan

>   		if (rc)
>   			goto failed;
>   	}
> +	pci_unlock_rescan_remove();
>   	return 0;
>   failed:
>   	while (i--)
>   		pci_iov_remove_virtfn(dev, i);
> +	pci_unlock_rescan_remove();
>   
>   	return rc;
>   }
> @@ -762,8 +765,10 @@ static void sriov_del_vfs(struct pci_dev *dev)
>   	struct pci_sriov *iov = dev->sriov;
>   	int i;
>   
> +	pci_lock_rescan_remove();
>   	for (i = 0; i < iov->num_VFs; i++)
>   		pci_iov_remove_virtfn(dev, i);
> +	pci_unlock_rescan_remove();
>   }
>   
>   static void sriov_disable(struct pci_dev *dev)
>