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