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

[David Lechner]

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.