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Date: Tue, 13 Feb 2024 13:45:56 -0600
From: Bjorn Helgaas <helgaas@kernel.org>
To: Leon Romanovsky <leonro@nvidia.com>
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
	Jim Harris <jim.harris@samsung.com>,
	Bjorn Helgaas <bhelgaas@google.com>, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, Jason Gunthorpe <jgg@nvidia.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Pierre =?utf-8?Q?Cr=C3=A9gut?= <pierre.cregut@orange.com>
Subject: Re: [PATCH v2 1/2] PCI/IOV: Revert "PCI/IOV: Serialize sysfs
 sriov_numvfs reads vs writes"
Message-ID: <20240213194556.GA1219770@bhelgaas>
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In-Reply-To: <20240213174602.GD52640@unreal>

On Tue, Feb 13, 2024 at 07:46:02PM +0200, Leon Romanovsky wrote:
> On Tue, Feb 13, 2024 at 09:59:54AM -0600, Bjorn Helgaas wrote:
> ...

> > I guess that means that if we apply this revert, the problem Pierre
> > reported will return.  Obviously the deadlock is more important than
> > the inconsistency Pierre observed, but from the user's point of view
> > this will look like a regression.
> > 
> > Maybe listening to netlink and then looking at sysfs isn't the
> > "correct" way to do this, but I don't want to just casually break
> > existing user code.  If we do contemplate doing the revert, at the
> > very least we should include specific details about what the user code
> > *should* do instead, at the level of the actual commands to use
> > instead of "ip monitor dev; cat ${path}/device/sriov_numvfs".
> 
> udevadm monitor will do the trick.
> 
> Another possible solution is to refactor the code to make sure that
> .probe on VFs happens only after sriov_numvfs is updated.

I like the idea of refactoring it so as to preserve the existing
ordering while also fixing the deadlock.

If, in addition, we want to update the sriov_numvfs documentation to
recommend "udevadm monitor" + /sys/.../sriov_numvfs, that's fine, too,
but I don't really like the idea of merging a patch and forcing users
to update their code.

So I'll table the v2 series for now and watch for a v3 with the
refactoring.

Bjorn