Re: [PATCH] counter: drop chrdev_lock

[David Lechner]

On 10/18/21 4:14 AM, William Breathitt Gray wrote:
> On Sun, Oct 17, 2021 at 01:55:21PM -0500, David Lechner wrote:
> > diff --git a/drivers/counter/counter-sysfs.c b/drivers/counter/counter-sysfs.c
> > index 1ccd771da25f..7bf8882ff54d 100644
> > --- a/drivers/counter/counter-sysfs.c
> > +++ b/drivers/counter/counter-sysfs.c
> > @@ -796,25 +796,18 @@ static int counter_events_queue_size_write(struct counter_device *counter,
> >   					   u64 val)
> >   {
> >   	DECLARE_KFIFO_PTR(events, struct counter_event);
> > -	int err = 0;
> > -
> > -	/* Ensure chrdev is not opened more than 1 at a time */
> > -	if (!atomic_add_unless(&counter->chrdev_lock, 1, 1))
> > -		return -EBUSY;
> > +	int err;
> >   
> >   	/* Allocate new events queue */
> >   	err = kfifo_alloc(&events, val, GFP_KERNEL);
> >   	if (err)
> > -		goto exit_early;
> > +		return err;
> >   
> >   	/* Swap in new events queue */
> >   	kfifo_free(&counter->events);
> >   	counter->events.kfifo = events.kfifo;
>
> Do we need to hold the events_lock mutex here for this swap in case
> counter_chrdev_read() is in the middle of reading the kfifo to
> userspace, or do the kfifo macros already protect us from a race
> condition here?
Another possibility might be to disallow changing the size while
events are enabled. Otherwise, we also need to protect against
write after free.

I considered this:

	swap(counter->events.kfifo, events.kfifo);
	kfifo_free(&events);

But I'm not sure that would be safe enough.