[PATCH] pci/iov: fix kobject_uevent() ordering in sriov_enable()

[PATCH] pci/iov: fix kobject_uevent() ordering in sriov_enable()

[Jim Harris]

Wait to call kobject_uevent() until all of the associated changes are done,
including updating the num_VFs value.

Suggested by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jim Harris <jim.harris@samsung.com>
---
 drivers/pci/iov.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
index 25dbe85c4217..3b768e20c7ab 100644
--- a/drivers/pci/iov.c
+++ b/drivers/pci/iov.c
@@ -683,8 +683,8 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
 	if (rc)
 		goto err_pcibios;
 
-	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
 	iov->num_VFs = nr_virtfn;
+	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
 
 	return 0;
 

Re: [PATCH] pci/iov: fix kobject_uevent() ordering in sriov_enable()

[Bjorn Helgaas]

On Wed, Dec 13, 2023 at 06:58:28PM +0000, Jim Harris wrote:
> Wait to call kobject_uevent() until all of the associated changes are done,
> including updating the num_VFs value.

This seems right to me.  Can we add a little rationale to the commit
log?  E.g., something about how num_VFs is visible to userspace via
sysfs and we don't want a race between (a) userspace reading num_VFs
because of KOBJ_CHANGE and (b) the kernel updating num_VFs?  (If
that's the actual reason.)

If there's a problem report about this, include that reference as
well.

> Suggested by: Leon Romanovsky <leonro@nvidia.com>
> Signed-off-by: Jim Harris <jim.harris@samsung.com>
> ---
>  drivers/pci/iov.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index 25dbe85c4217..3b768e20c7ab 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -683,8 +683,8 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
>  	if (rc)
>  		goto err_pcibios;
>  
> -	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
>  	iov->num_VFs = nr_virtfn;
> +	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
>  
>  	return 0;
>  
> 

Re: [PATCH] pci/iov: fix kobject_uevent() ordering in sriov_enable()

[Jim Harris]

On Wed, Dec 13, 2023 at 01:26:52PM -0600, Bjorn Helgaas wrote:
> On Wed, Dec 13, 2023 at 06:58:28PM +0000, Jim Harris wrote:
> > Wait to call kobject_uevent() until all of the associated changes are done,
> > including updating the num_VFs value.
> 
> This seems right to me.  Can we add a little rationale to the commit
> log?  E.g., something about how num_VFs is visible to userspace via
> sysfs and we don't want a race between (a) userspace reading num_VFs
> because of KOBJ_CHANGE and (b) the kernel updating num_VFs?  (If
> that's the actual reason.)
> 
> If there's a problem report about this, include that reference as
> well.

There's a second patch that I haven't pushed yet that has more of the
rationale that led to this suggestion from Leon - see thread
"Locking between vfio hot-remove..." I'll push the two patches as a series.

The second patch will effectively revert 35ff867b7, since Leon felt the
reasoning behind that patch was incorrect. I'll tie all of that into the
cover letter.

-Jim

> 
> > Suggested by: Leon Romanovsky <leonro@nvidia.com>
> > Signed-off-by: Jim Harris <jim.harris@samsung.com>
> > ---
> >  drivers/pci/iov.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > index 25dbe85c4217..3b768e20c7ab 100644
> > --- a/drivers/pci/iov.c
> > +++ b/drivers/pci/iov.c
> > @@ -683,8 +683,8 @@ static int sriov_enable(struct pci_dev *dev, int nr_virtfn)
> >  	if (rc)
> >  		goto err_pcibios;
> >  
> > -	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
> >  	iov->num_VFs = nr_virtfn;
> > +	kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
> >  
> >  	return 0;
> >  
> >