Subject: Re: [PATCH net v4] nfc: nfcmrvl: main: reorder destructive operations in nfcmrvl_nci_unregister_dev to avoid bugs

From: Jakub Kicinski <[email-masked]>
Date: Wed, 27 Apr 2022 17:45:48 -0700

On Wed, 27 Apr 2022 09:14:38 +0800 Duoming Zhou wrote:
> diff --git a/net/nfc/core.c b/net/nfc/core.c
> index dc7a2404efd..1d91334ee86 100644
> --- a/net/nfc/core.c
> +++ b/net/nfc/core.c
> @@ -25,6 +25,8 @@
>  #define NFC_CHECK_PRES_FREQ_MS	2000
>  
>  int nfc_devlist_generation;
> +/* nfc_download: used to judge whether nfc firmware download could start */
> +static bool nfc_download;
>  DEFINE_MUTEX(nfc_devlist_mutex);
>  
>  /* NFC device ID bitmap */
> @@ -38,7 +40,7 @@ int nfc_fw_download(struct nfc_dev *dev, const char *firmware_name)
>  
>  	device_lock(&dev->dev);
>  
> -	if (!device_is_registered(&dev->dev)) {
> +	if (!device_is_registered(&dev->dev) || !nfc_download) {
>  		rc = -ENODEV;
>  		goto error;
>  	}
> @@ -1134,6 +1136,7 @@ int nfc_register_device(struct nfc_dev *dev)
>  			dev->rfkill = NULL;
>  		}
>  	}
> +	nfc_download = true;
>  	device_unlock(&dev->dev);
>  
>  	rc = nfc_genl_device_added(dev);
> @@ -1166,6 +1169,7 @@ void nfc_unregister_device(struct nfc_dev *dev)
>  		rfkill_unregister(dev->rfkill);
>  		rfkill_destroy(dev->rfkill);
>  	}
> +	nfc_download = false;
>  	device_unlock(&dev->dev);
>  
>  	if (dev->ops->check_presence) {

You can't use a single global variable, there can be many devices 
each with their own lock.

Paolo suggested adding a lock, if spin lock doesn't fit the bill
why not add a mutex?