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

[Leon Romanovsky]

On Fri, Feb 09, 2024 at 07:20:28PM -0800, Kuppuswamy Sathyanarayanan wrote:
> 
> 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.

It was never an issue, the idea that sysfs read and write should be serialized by kernel
is not correct by definition. 

Thanks

> 
> >
> > 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
>