linux-kernel - Re: [RFC PATCH] uio_hv_generic: Fix sysfs creation path for ring buffer

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Message-ID: <e35ecae5-1596-41f6-92f2-62a79d4d31d6@linux.microsoft.com>
Date: Mon, 17 Feb 2025 09:35:33 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: Stephen Hemminger <stephen@...workplumber.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "K . Y . Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org, stable@...nel.org,
 Saurabh Sengar <ssengar@...ux.microsoft.com>
Subject: Re: [RFC PATCH] uio_hv_generic: Fix sysfs creation path for ring
 buffer



On 2/14/2025 10:41 PM, Stephen Hemminger wrote:
> On Fri, 14 Feb 2025 08:41:57 +0100
> Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> 
>> On Fri, Feb 14, 2025 at 12:35:44PM +0530, Naman Jain wrote:
>>>
>>>
>>> On 2/14/2025 12:21 PM, Greg Kroah-Hartman wrote:
>>>> On Fri, Feb 14, 2025 at 12:13:51PM +0530, Naman Jain wrote:
>>>>> On regular bootup, devices get registered to vmbus first, so when
>>>>> uio_hv_generic driver for a particular device type is probed,
>>>>> the device is already initialized and added, so sysfs creation in
>>>>> uio_hv_generic probe works fine. However, when device is removed
>>>>> and brought back, the channel rescinds and again gets registered
>>>>> to vmbus. However this time, the uio_hv_generic driver is already
>>>>> registered to probe for that device and in this case sysfs creation
>>>>> is tried before the device gets initialized completely. Fix this by
>>>>> deferring sysfs creation till device gets initialized completely.
>>>>>
>>>>> Problem path:
>>>>> vmbus_device_register
>>>>>       device_register
>>>>>           uio_hv_generic probe
>>>>> 		    sysfs_create_bin_file (fails here)
>>>>
>>>> Ick, that's the issue, you shouldn't be manually creating sysfs files.
>>>> Have the driver core do it for you at the proper time, which should make
>>>> your logic much simpler, right?
>>>>
>>>> Set the default attribute groups instead of manually creating this and
>>>> see if that works out better.
>>>>
>>>> thanks,
>>>>
>>>> greg k-h
>>>
>>> Thanks for reviewing Greg. I tried this approach and here are my
>>> observations:
>>>
>>> What I could create with ATTRIBUTE_GROUPS:
>>> /sys/bus/vmbus/devices/eb765408-105f-49b6-b4aa-c123b64d17d4/ring
>>>
>>> The one we have right now:
>>> /sys/bus/vmbus/devices/eb765408-105f-49b6-b4aa-c123b64d17d4/channels/6/ring
>>
>> What is "channels" and "6" here?  Are they real devices or just a
>> directory name or something else?
>>
>>> I could not find a way to tweak attributes to create the "ring" under above
>>> path. I could see the variations of sys_create_* which provides a
>>> way to pass kobj and do that, but that is something we are already
>>> using.
>>
>> No driver should EVER be pointing to a raw kobject, that's a huge hint
>> that something is really wrong.  Also, if a raw kobject is in a device
>> path in the middle like this, it will not be seen properly from
>> userspace library tools :(
>>
>> So again, what is creating the "channels" and "6" subdirectories?  All
>> of that shoudl be under full control by the uio device, right?
> 
> The original design of exposing channels was based on what the
> network core does to expose queues. Worth comparing the two
> to see if there is any shared insight.

Thanks Greg and Stephen. I'll try to find it.

Regards,
Naman