Hello,
I have some questions about the locking techniques used inside
file drivers/staging/most/hdm-usb/hdm_usb.c.
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)?
Thank you very much in advance.
With best regards
Michael
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