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

From: Bjorn Helgaas <helgaas@kernel.org>
To: CREGUT Pierre IMT/OLN <pierre.cregut@orange.com>
Cc: linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	Donald Dutile <ddutile@redhat.com>,
	Alexander Duyck <alexander.h.duyck@intel.com>,
	Jakub Kicinski <jakub.kicinski@netronome.com>
Subject: Re: [PATCH] PCI/IOV: update num_VFs earlier
Date: Tue, 8 Oct 2019 16:38:35 -0500	[thread overview]
Message-ID: <20191008213835.GA230403@google.com> (raw)
In-Reply-To: <20191003221007.GA209602@google.com>

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
your great patience!

> commit 0940fc95da45
> Author: Pierre Crégut <pierre.cregut@orange.com>
> 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@orange.com
>     Signed-off-by: Pierre Crégut <pierre.cregut@orange.com>
>     Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> 
> 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);
> +	num_vfs = pdev->sriov->num_VFs;
> +	device_lock(&pdev->dev);
>  
> -	return sprintf(buf, "%u\n", pdev->sriov->num_VFs);
> +	return sprintf(buf, "%u\n", num_vfs);
>  }
>  
>  /*