linux-kernel - Re: [PATCH v2 1/2] PCI/IOV: Revert "PCI/IOV: Serialize sysfs sriov

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Date: Tue, 13 Feb 2024 10:40:28 +0100
From: pierre.cregut@...nge.com
To: Jim Harris <jim.harris@...sung.com>, Bjorn Helgaas <helgaas@...nel.org>
CC: Leon Romanovsky <leonro@...dia.com>, Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, 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>, Jason Gunthorpe <jgg@...dia.com>, Alex
 Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 1/2] PCI/IOV: Revert "PCI/IOV: Serialize sysfs
 sriov_numvfs reads vs writes"

First sorry for not answering earlier but it is a long time ago. I do
not work on the topic any more (a monitoring tool Skydive, an open
source project no more actively developed as far as I know) and only
have vague memories of it.

> 
>      kobject_uevent(&dev->dev.kobj, KOBJ_CHANGE);
>      iov->num_VFs = nr_virtfn;
> 
> which is identical to how the code looks today. Is it possible that
> userspace could react to this uevent and read the stale num_VFs before
> iov->num_VFs gets written here? I mean, theoretically it's possible, but
> from the bug report it seems like the scenario Pierre was facing was
> 100% reproducible.
 From my memories yes that was exactly the problem. Any stable method
that could detect the change of configuration in user land and ensure
that we get a reliable value of num_vfs after we received it would be
fine.

Best regards,

Pierre Crégut

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