On Mon, Jul 18, 2016 at 08:14:47PM +0200, mhornung.linux@xxxxxxxxx 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
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