linux-kernel - Re: [PATCH] pci/iov: fix kobject_uevent() ordering in sriov

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Date: Fri, 15 Dec 2023 22:11:17 +0000
From: Jim Harris <jim.harris@...sung.com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: Bjorn Helgaas <bhelgaas@...gle.com>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Leon Romanovsky <leonro@...dia.com>, "Jason
 Gunthorpe" <jgg@...dia.com>, Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH] pci/iov: fix kobject_uevent() ordering in
 sriov_enable()

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@...dia.com>
> > Signed-off-by: Jim Harris <jim.harris@...sung.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;
> >  
> >