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

[Kuppuswamy Sathyanarayanan]

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@xxxxxxxxxx>
> Reviewed-by: Leon Romanovsky <leonro@xxxxxxxxxx>
> Signed-off-by: Jim Harris <jim.harris@xxxxxxxxxxx>
> ---
>  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