Re: [PATCH] PCI/IOV: update num_VFs earlier

[Bjorn Helgaas]

On Tue, Oct 08, 2019 at 06:06:46PM -0400, Don Dutile wrote:
> On 10/08/2019 05:38 PM, Bjorn Helgaas wrote:
> > On Thu, Oct 03, 2019 at 05:10:07PM -0500, Bjorn Helgaas wrote:
> > > On Thu, Oct 03, 2019 at 11:04:45AM +0200, CREGUT Pierre IMT/OLN wrote:
> > > > ...
> > 
> > > > NIC drivers send netlink events when their state change, but it is
> > > > the core that changes the value of num_vfs. So I would think it is
> > > > the core responsibility to make sure the exposed value makes sense
> > > > and it would be better to ignore the details of the driver
> > > > implementation.
> > > 
> > > Yes, I think you're right.  And I like your previous suggestion of
> > > just locking the device in the reader.  I'm not enough of a sysfs
> > > expert to know if there's a good reason to avoid a lock there.  Does
> > > the following look reasonable to you?
> > 
> > I applied the patch below to pci/virtualization for v5.5, thanks for
> I hope not... see below
> 
> > your great patience!
> > 
> > > commit 0940fc95da45
> > > Author: Pierre Crégut <pierre.cregut@xxxxxxxxxx>
> > > Date:   Wed Sep 11 09:27:36 2019 +0200
> > > 
> > >      PCI/IOV: Serialize sysfs sriov_numvfs reads vs writes
> > >      When sriov_numvfs is being updated, drivers may notify about new devices
> > >      before they are reflected in sriov->num_VFs, so concurrent sysfs reads
> > >      previously returned stale values.
> > >      Serialize the sysfs read vs the write so the read returns the correct
> > >      num_VFs value.
> > >      Link: https://bugzilla.kernel.org/show_bug.cgi?id=202991
> > >      Link: https://lore.kernel.org/r/20190911072736.32091-1-pierre.cregut@xxxxxxxxxx
> > >      Signed-off-by: Pierre Crégut <pierre.cregut@xxxxxxxxxx>
> > >      Signed-off-by: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> > > 
> > > diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> > > index b3f972e8cfed..e77562aabbae 100644
> > > --- a/drivers/pci/iov.c
> > > +++ b/drivers/pci/iov.c
> > > @@ -254,8 +254,14 @@ 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);
>                ^^^^^ lock
> > > +	num_vfs = pdev->sriov->num_VFs;
> > > +	device_lock(&pdev->dev);
>                ^^^^ and lock again!

Oops, sorry, my fault.  Fixed.

> > > -	return sprintf(buf, "%u\n", pdev->sriov->num_VFs);
> > > +	return sprintf(buf, "%u\n", num_vfs);
> > >   }
> > >   /*
>