drivers: staging: most: Locking question

[greg@kroah.com (Greg KH)]

On Mon, Jul 18, 2016 at 08:14:47PM +0200, mhornung.linux at gmail.com wrote:
> Hello,
> 
> I have some questions about the locking techniques used inside
> file drivers/staging/most/hdm-usb/hdm_usb.c.

Eeek, never use anything from drivers/staging/ as a good example of
anything.  Please, that code is in staging for a good reason, only worry
about code outside of drivers/staging/ for good examples.

> 
> The one and only call to function free_anchored_buffers is locked by a Mutex:
> 
> ----------------------------------------------------------------------------
>     	...
>         mutex_lock(&mdev->io_mutex);
> 	free_anchored_buffers(mdev, channel);
> 	if (mdev->padding_active[channel])
> 		mdev->padding_active[channel] = false;
> 
>         if (mdev->conf[channel].data_type == MOST_CH_ASYNC) {
> 	        del_timer_sync(&mdev->link_stat_timer);
> 		cancel_work_sync(&mdev->poll_work_obj);
> 	}
> 	mutex_unlock(&mdev->io_mutex);
> 	...						
> ----------------------------------------------------------------------------
> 
> Then, inside function free_anchored_buffers, they use a (from my point of view)
> somewhat complex spinlock variant:
> 
> ----------------------------------------------------------------------------
> static void free_anchored_buffers(struct most_dev *mdev, unsigned int channel)
> {
> 	struct mbo *mbo;
> 	struct buf_anchor *anchor, *tmp;
> 	unsigned long flags;
> 
> 	spin_lock_irqsave(&mdev->anchor_list_lock[channel], flags);
> 	list_for_each_entry_safe(anchor, tmp, &mdev->anchor_list[channel],
> 				 list) {
> 		struct urb *urb = anchor->urb;
> 
> 		spin_unlock_irqrestore(&mdev->anchor_list_lock[channel], flags);
> 		if (likely(urb)) {
> 			mbo = urb->context;
> 			if (!irqs_disabled()) {
> 				usb_kill_urb(urb);
> 			} else {
> 				usb_unlink_urb(urb);
> 				wait_for_completion(&anchor->urb_compl);
> 			}
> 			if ((mbo) && (mbo->complete)) {
> 				mbo->status = MBO_E_CLOSE;
> 				mbo->processed_length = 0;
> 				mbo->complete(mbo);
> 			}
> 			usb_free_urb(urb);
> 		}
> 		spin_lock_irqsave(&mdev->anchor_list_lock[channel], flags);
> 		list_del(&anchor->list);
> 		cancel_work_sync(&anchor->clear_work_obj);
> 		kfree(anchor);
> 	}
> 	spin_unlock_irqrestore(&mdev->anchor_list_lock[channel], flags);
> }
> ----------------------------------------------------------------------------
> 
> To my questions:
> 
> #1: What is the intention of locking a whole function with a Mutex and then
>     using spinlocks inside the function? Wouldn't it be sufficient to use
>     one locking technique?
> #2: Why is the spinlock not just locking the whole list_for_each_entry part or
>     just the list_del(&anchor->list)?

Why not ask the authors and maintainers of these files?  They would be
the best to answer them, don't you think?

thanks,

greg k-h