Re: [PATCH v2 1/2] PCI/IOV: Revert "PCI/IOV: Serialize sysfs sriov_numvfs reads vs writes"

Kuppuswamy Sathyanarayanan
2024-02-10T03:20:29Z

On 2/9/24 3:52 PM, Jim Harris wrote:
> If an SR-IOV enabled device is held by vfio, and the device is removed,
> vfio will hold device lock and notify userspace of the removal. If
> userspace reads the sriov_numvfs sysfs entry, that thread will be blocked
> since sriov_numvfs_show() also tries to acquire the device lock. If that
> same thread is responsible for releasing the device to vfio, it results in
> a deadlock.
>
> The proper way to detect a change to the num_VFs value is to listen for a
> sysfs event, not to add a device_lock() on the attribute _show() in the
> kernel.

Since you are reverting a commit that synchronizes SysFS read
/write, please add some comments about why it is not an
issue anymore.

>
> This reverts commit 35ff867b76576e32f34c698ccd11343f7d616204.
> Revert had a small conflict, the sprintf() is now changed to sysfs_emit().
>
> Link: https://lore.kernel.org/linux-pci/ZXJI5+f8bUelVXqu@ubuntu/
> Suggested-by: Leon Romanovsky <leonro@nvidia.com>
> Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
> Signed-off-by: Jim Harris <jim.harris@samsung.com>
> ---
> drivers/pci/iov.c | 8 +-------
> 1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index aaa33e8dc4c9..0ca20cd518d5 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -395,14 +395,8 @@ static ssize_t sriov_numvfs_show(struct device *dev,
> char *buf)
> {
> struct pci_dev *pdev = to_pci_dev(dev);
> - u16 num_vfs;
> -
> - /* Serialize vs sriov_numvfs_store() so readers see valid num_VFs */
> - device_lock(&pdev->dev);
> - num_vfs = pdev->sriov->num_VFs;
> - device_unlock(&pdev->dev);
>
> - return sysfs_emit(buf, "%u\n", num_vfs);
> + return sysfs_emit(buf, "%u\n", pdev->sriov->num_VFs);
> }
>
> /*
>
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer