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

[David Lechner <david@lechnology.com>]

On 10/25/20 8:18 AM, William Breathitt Gray wrote:
> On Tue, Oct 20, 2020 at 11:06:42AM -0500, David Lechner wrote:
>> 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.
> 
> You're right, it does say in the kfifo.h comments that with only one
> concurrent reader and one current write, we don't need extra locking to
> use these macros. Because we only have one kfifo_to_user() operating on
> counter->events, does that mean we don't need locking at all here for
> the counter_chrdev_read() function?
> 
> William Breathitt Gray
> 

Even if we have the policy that only one file handle to the chrdev
can be open at a time, it is still possible that the it could be
read from multiple threads. So it I think it makes sense to keep
it just to be safe.