Re: [PATCH v5 3/5] counter: Add character device interface

William Breathitt Gray
2020-10-18T16:59:49Z

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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.

William Breathitt Gray

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