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Date:   Fri, 29 Apr 2022 09:19:36 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Duoming Zhou <duoming@....edu.cn>, linux-kernel@...r.kernel.org,
        kuba@...nel.org, gregkh@...uxfoundation.org
Cc:     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 29/04/2022 03:14, 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.
> 
>  include/net/nfc/nfc.h |  1 +
>  net/nfc/core.c        | 26 ++++++++++++++------------
>  2 files changed, 15 insertions(+), 12 deletions(-)
> 
> diff --git a/include/net/nfc/nfc.h b/include/net/nfc/nfc.h
> index 5dee575fbe8..7bb4ccb1830 100644
> --- a/include/net/nfc/nfc.h
> +++ b/include/net/nfc/nfc.h
> @@ -167,6 +167,7 @@ struct nfc_dev {
>  	int n_targets;
>  	int targets_generation;
>  	struct device dev;
> +	bool dev_register;
>  	bool dev_up;
>  	bool fw_download_in_progress;
>  	u8 rf_mode;
> diff --git a/net/nfc/core.c b/net/nfc/core.c
> index dc7a2404efd..52147da2286 100644
> --- a/net/nfc/core.c
> +++ b/net/nfc/core.c
> @@ -38,7 +38,7 @@ int nfc_fw_download(struct nfc_dev *dev, const char *firmware_name)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -94,7 +94,7 @@ int nfc_dev_up(struct nfc_dev *dev)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -142,7 +142,7 @@ int nfc_dev_down(struct nfc_dev *dev)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -207,7 +207,7 @@ int nfc_start_poll(struct nfc_dev *dev, u32 im_protocols, u32 tm_protocols)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -246,7 +246,7 @@ int nfc_stop_poll(struct nfc_dev *dev)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -291,7 +291,7 @@ int nfc_dep_link_up(struct nfc_dev *dev, int target_index, u8 comm_mode)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -335,7 +335,7 @@ int nfc_dep_link_down(struct nfc_dev *dev)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -401,7 +401,7 @@ int nfc_activate_target(struct nfc_dev *dev, u32 target_idx, u32 protocol)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -448,7 +448,7 @@ int nfc_deactivate_target(struct nfc_dev *dev, u32 target_idx, u8 mode)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -495,7 +495,7 @@ int nfc_data_exchange(struct nfc_dev *dev, u32 target_idx, struct sk_buff *skb,
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		kfree_skb(skb);
>  		goto error;
> @@ -552,7 +552,7 @@ int nfc_enable_se(struct nfc_dev *dev, u32 se_idx)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -601,7 +601,7 @@ int nfc_disable_se(struct nfc_dev *dev, u32 se_idx)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!dev->dev_register) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -1134,6 +1134,7 @@ int nfc_register_device(struct nfc_dev *dev)
>  			dev->rfkill = NULL;
>  		}
>  	}
> +	dev->dev_register = true;
>  	device_unlock(&dev->dev);
>  
>  	rc = nfc_genl_device_added(dev);
> @@ -1166,6 +1167,7 @@ void nfc_unregister_device(struct nfc_dev *dev)
>  		rfkill_unregister(dev->rfkill);
>  		rfkill_destroy(dev->rfkill);
>  	}
> +	dev->dev_register = false;

We already have flag for it - dev->shutting_down. Currently it is used
only in if device implements check_presence but I think it can be easily
moved to common path.

Having multiple fields for similar, but slightly different cases, is
getting us closer and closer to spaghetti code.


Best regards,
Krzysztof

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