Re: [PATCH 1/2] Bluetooth: bcm203x: Fix race condition on disconnect

[Marcel Holtmann <marcel@holtmann.org>]

Hi David,

> When disconnecting a bcm203x device we kill and destroy the usb-urb, however,
> there might still be a pending work-structure which resubmits the now invalid
> urb. To avoid this race condition, we simply set a shutdown-flag and
> synchronously kill the worker first.
> 
> This also adds a comment to all schedule_work()s, as it is really not clear
> that they are used as replacement for short timers (which can be seen in the git
> history).
> 
> Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
> ---
>  drivers/bluetooth/bcm203x.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/bluetooth/bcm203x.c b/drivers/bluetooth/bcm203x.c
> index 8b1b643..6b42732 100644
> --- a/drivers/bluetooth/bcm203x.c
> +++ b/drivers/bluetooth/bcm203x.c
> @@ -24,6 +24,7 @@
>  
>  #include <linux/module.h>
>  
> +#include <linux/atomic.h>
>  #include <linux/kernel.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
> @@ -65,6 +66,7 @@ struct bcm203x_data {
>  	unsigned long		state;
>  
>  	struct work_struct	work;
> +	atomic_t		shutdown;
>  
>  	struct urb		*urb;
>  	unsigned char		*buffer;
> @@ -97,6 +99,7 @@ static void bcm203x_complete(struct urb *urb)
>  
>  		data->state = BCM203X_SELECT_MEMORY;
>  
> +		/* use workqueue to have a small delay */
>  		schedule_work(&data->work);
>  		break;
>  
> @@ -155,6 +158,10 @@ static void bcm203x_work(struct work_struct *work)
>  	struct bcm203x_data *data =
>  		container_of(work, struct bcm203x_data, work);
>  
> +	smp_rmb();
> +	if (atomic_read(&data->shutdown))
> +		return;
> +
>  	if (usb_submit_urb(data->urb, GFP_ATOMIC) < 0)
>  		BT_ERR("Can't submit URB");
>  }
> @@ -243,6 +250,7 @@ static int bcm203x_probe(struct usb_interface *intf, const struct usb_device_id
>  
>  	usb_set_intfdata(intf, data);
>  
> +	/* use workqueue to have a small delay */
>  	schedule_work(&data->work);
>  
>  	return 0;
> @@ -254,6 +262,10 @@ static void bcm203x_disconnect(struct usb_interface *intf)
>  
>  	BT_DBG("intf %p", intf);
>  
> +	atomic_inc(&data->shutdown);
> +	smp_wmb();
> +	cancel_work_sync(&data->work);
> +

can you quickly explain to me why we need the memory barrier here.

Regards

Marcel