Re: [PATCH net v4] nfc: ... device_is_registered() is data race-able

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Apr 28, 2022 at 04:49:18PM +0800, Lin Ma wrote:
> Hello Greg,
> 
> > 
> > It shouldn't be, if you are using it properly :)
> > 
> > [...]
> > 
> > Yes, you should almost never use that call.  Seems the nfc subsystem is
> > the most common user of it for some reason :(
> 
> Cool, and I believe that the current nfc core code does not use it properly. :(
> 
> > 
> > What state are you trying to track here exactly?
> > 
> 
> Forget about the firmware downloading race that raised by Duoming in this channel,
> all the netlink handler code in net/nfc/core.c depends on the device_is_registered
> macro.
> 
> My idea is to introduce a patch like below:
> 
>  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 5dee575fbe86..d84e53802b06 100644
> --- a/include/net/nfc/nfc.h
> +++ b/include/net/nfc/nfc.h
> @@ -168,6 +168,7 @@ struct nfc_dev {
>  	int targets_generation;
>  	struct device dev;
>  	bool dev_up;
> +	bool dev_register;
>  	bool fw_download_in_progress;
>  	u8 rf_mode;
>  	bool polling;
> diff --git a/net/nfc/core.c b/net/nfc/core.c
> index dc7a2404efdf..208e6bb0804e 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;
>  	}
> 
> [...]
> 
> @@ -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);
> @@ -1162,6 +1163,7 @@ void nfc_unregister_device(struct nfc_dev *dev)
>  			 "was removed\n", dev_name(&dev->dev));
>  
>  	device_lock(&dev->dev);
> +	dev->dev_register = false;
>  	if (dev->rfkill) {
>  		rfkill_unregister(dev->rfkill);
>  		rfkill_destroy(dev->rfkill);
> -- 
> 2.35.1
> 
> The added dev_register variable can function like the original device_is_registered and does not race-able
> because of the protection of device_lock.

Yes, that looks better, but what is the root problem here that you are
trying to solve?  Why does NFC need this when no other subsystem does?

thansk,

greg k-h