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

[David Lechner]

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.