linux-kernel - Re: [PATCH net v4] nfc: ... device_is

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Date:   Thu, 28 Apr 2022 10:16:36 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Lin Ma <linma@....edu.cn>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Duoming Zhou <duoming@....edu.cn>,
        krzysztof.kozlowski@...aro.org, pabeni@...hat.com,
        linux-kernel@...r.kernel.org, davem@...emloft.net,
        alexander.deucher@....com, akpm@...ux-foundation.org,
        broonie@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net v4] nfc: ... device_is_registered() is data race-able

On Thu, Apr 28, 2022 at 03:55:01PM +0800, Lin Ma wrote:
> Hello Greg,
> 
> 
> > 
> > You should not be making these types of checks outside of the driver
> > core.
> > 
> > > This is by no means matching our expectations as one of our previous patch relies on the device_is_registered code.
> > 
> > Please do not do that.
> > 
> > > 
> > > -> the patch: 3e3b5dfcd16a ("NFC: reorder the logic in nfc_{un,}register_device")
> > > 
> > <...>
> > > 
> > > In another word, the device_del -> kobject_del -> __kobject_del is not protected by the device_lock.
> > 
> > Nor should it be.
> > 
> 
> I may have mistakenly presented my point. In fact, there is nothing wrong with the device core, nothing to do with the internal of device_del and device_is_registered implementation. And, of course, we will not add any code or do any modification to the device/driver base code.
> 
> The point is the combination of device_is_registered + device_del, which is used in NFC core, is not safe.

It shouldn't be, if you are using it properly :)

> That is to say, even the device_is_registered can return True even the device_del is executing in another thread.

Yes, you should almost never use that call.  Seems the nfc subsystem is
the most common user of it for some reason :(

> (By debugging we think this is true, correct me if it is not)
> 
> Hence we want to add additional state in nfc_dev object to fix that, not going to add any state in device/driver core.

What state are you trying to track here exactly?

thanks,

greg k-h