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Subject: Re: [PATCH] PCI/IOV: update num_VFs earlier
To:     Bjorn Helgaas <helgaas@kernel.org>,
        CREGUT Pierre IMT/OLN <pierre.cregut@orange.com>
Cc:     linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        Alexander Duyck <alexander.h.duyck@intel.com>,
        Jakub Kicinski <jakub.kicinski@netronome.com>
References: <20191008213835.GA230403@google.com>
From:   Don Dutile <ddutile@redhat.com>
Message-ID: <4ac38c7a-b8d4-9a1c-10ab-ce8b4cf2991f@redhat.com>
Date:   Tue, 8 Oct 2019 18:06:46 -0400
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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!
>>   
>> -	return sprintf(buf, "%u\n", pdev->sriov->num_VFs);
>> +	return sprintf(buf, "%u\n", num_vfs);
>>   }
>>   
>>   /*