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Date:   Fri, 29 Apr 2022 17:18:36 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org>
Cc:     linux-kernel@...r.kernel.org, kuba@...nel.org,
        gregkh@...uxfoundation.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: Re: [PATCH net v5 1/2] nfc: replace improper check
 device_is_registered() in netlink related functions

Hello,

On Fri, 29 Apr 2022 09:19:36 +0200 Krzysztof 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.

Thanks a lot for your suggestion, I will move dev->shutting_down to
common path.

Best regards,
Duoming Zhou

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