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Date:   Fri, 29 Apr 2022 09:03:01 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Duoming Zhou <duoming@....edu.cn>
Cc:     linux-kernel@...r.kernel.org, kuba@...nel.org,
        krzysztof.kozlowski@...aro.org, davem@...emloft.net,
        edumazet@...gle.com, pabeni@...hat.com, alexander.deucher@....com,
        akpm@...ux-foundation.org, broonie@...nel.org,
        netdev@...r.kernel.org, linma@....edu.cn
Subject: Re: [PATCH net v5 1/2] nfc: replace improper check
 device_is_registered() in netlink related functions

On Fri, Apr 29, 2022 at 09:14:32AM +0800, Duoming Zhou wrote:
> The device_is_registered() in nfc core is used to check whether
> nfc device is registered in netlink related functions such as
> nfc_fw_download(), nfc_dev_up() and so on. Although device_is_registered()
> is protected by device_lock, there is still a race condition between
> device_del() and device_is_registered(). The root cause is that
> kobject_del() in device_del() is not protected by device_lock.
> 
>    (cleanup task)         |     (netlink task)
>                           |
> nfc_unregister_device     | nfc_fw_download
>  device_del               |  device_lock
>   ...                     |   if (!device_is_registered)//(1)
>   kobject_del//(2)        |   ...
>  ...                      |  device_unlock
> 
> The device_is_registered() returns the value of state_in_sysfs and
> the state_in_sysfs is set to zero in kobject_del(). If we pass check in
> position (1), then set zero in position (2). As a result, the check
> in position (1) is useless.
> 
> This patch uses bool variable instead of device_is_registered() to judge
> whether the nfc device is registered, which is well synchronized.
> 
> Fixes: 3e256b8f8dfa ("NFC: add nfc subsystem core")
> Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> ---
> Changes in v5:
>   - Replace device_is_registered() to bool variable.

Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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