On 10/18/20 11:58 AM, William Breathitt Gray wrote:
On Wed, Oct 14, 2020 at 05:40:44PM -0500, David Lechner wrote:
On 9/26/20 9:18 PM, William Breathitt Gray wrote:
+static ssize_t counter_chrdev_read(struct file *filp, char __user *buf,
+ size_t len, loff_t *f_ps)
+{
+ struct counter_device *const counter = filp->private_data;
+ int err;
+ unsigned long flags;
+ unsigned int copied;
+
+ if (len < sizeof(struct counter_event))
+ return -EINVAL;
+
+ do {
+ if (kfifo_is_empty(&counter->events)) {
+ if (filp->f_flags & O_NONBLOCK)
+ return -EAGAIN;
+
+ err = wait_event_interruptible(counter->events_wait,
+ !kfifo_is_empty(&counter->events));
+ if (err)
+ return err;
+ }
+
+ raw_spin_lock_irqsave(&counter->events_lock, flags);
+ err = kfifo_to_user(&counter->events, buf, len, &copied);
+ raw_spin_unlock_irqrestore(&counter->events_lock, flags);
+ if (err)
+ return err;
+ } while (!copied);
+
+ return copied;
+}
All other uses of kfifo_to_user() I saw use a mutex instead of spin
lock. I don't see a reason for disabling interrupts here.
The Counter character device interface is special in this case because
counter->events could be accessed from an interrupt context. This is
possible if counter_push_event() is called for an interrupt (as is the
case for the 104_quad_8 driver). In this case, we can't use mutex
because we can't sleep in an interrupt context, so our only option is to
use spin lock.
The way I understand it, locking is only needed for concurrent readers
and locking between reader and writer is not needed.
