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

[Don Dutile]

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