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    SubjectRe: [PATCH v5 3/5] counter: Add character device interface
    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
    [unhandled content-type:application/pgp-signature]
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