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Subject: Re: [PATCH] PCI/IOV: update num_VFs earlier
To:     Bjorn Helgaas <helgaas@kernel.org>
Cc:     CREGUT Pierre IMT/OLN <pierre.cregut@orange.com>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alexander Duyck <alexander.h.duyck@intel.com>,
        Jakub Kicinski <jakub.kicinski@netronome.com>
References: <20191009123135.GA62790@google.com>
From:   Don Dutile <ddutile@redhat.com>
Message-ID: <4b08c218-790b-2f26-a5a0-b66a4d4e67e9@redhat.com>
Date:   Wed, 9 Oct 2019 10:20:33 -0400
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On 10/09/2019 08:31 AM, Bjorn Helgaas wrote:
> 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@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);
>>                 ^^^^^ lock
>>>> +	num_vfs = pdev->sriov->num_VFs;
>>>> +	device_lock(&pdev->dev);
>>                 ^^^^ and lock again!
> 
> Oops, sorry, my fault.  Fixed.
> 
Thanks.
--dd

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