Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute

[William Breathitt Gray]

On Fri, Feb 26, 2021 at 06:14:12PM -0600, David Lechner wrote:
> On 2/25/21 6:03 PM, William Breathitt Gray wrote:
> > On Sun, Feb 21, 2021 at 03:51:40PM +0000, Jonathan Cameron wrote:
> >> On Thu, 18 Feb 2021 19:32:16 +0900
> >> William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
> >>
> >>> On Sun, Feb 14, 2021 at 06:11:46PM +0000, Jonathan Cameron wrote:
> >>>> On Fri, 12 Feb 2021 21:13:44 +0900
> >>>> William Breathitt Gray <vilhelm.gray@xxxxxxxxx> wrote:
> >>>>    
> >>>>> The events_queue_size sysfs attribute provides a way for users to
> >>>>> dynamically configure the Counter events queue size for the Counter
> >>>>> character device interface. The size is in number of struct
> >>>>> counter_event data structures. The number of elements will be rounded-up
> >>>>> to a power of 2 due to a requirement of the kfifo_alloc function called
> >>>>> during reallocation of the queue.
> >>>>>
> >>>>> Cc: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
> >>>>> Signed-off-by: William Breathitt Gray <vilhelm.gray@xxxxxxxxx>
> >>>>> ---
> >>>>>   Documentation/ABI/testing/sysfs-bus-counter |  8 +++++++
> >>>>>   drivers/counter/counter-chrdev.c            | 23 +++++++++++++++++++
> >>>>>   drivers/counter/counter-chrdev.h            |  2 ++
> >>>>>   drivers/counter/counter-sysfs.c             | 25 +++++++++++++++++++++
> >>>>>   4 files changed, 58 insertions(+)
> >>>>>
> >>>>> diff --git a/Documentation/ABI/testing/sysfs-bus-counter b/Documentation/ABI/testing/sysfs-bus-counter
> >>>>> index 847e96f19d19..f6cb2a8b08a7 100644
> >>>>> --- a/Documentation/ABI/testing/sysfs-bus-counter
> >>>>> +++ b/Documentation/ABI/testing/sysfs-bus-counter
> >>>>> @@ -212,6 +212,14 @@ Description:
> >>>>>   		both edges:
> >>>>>   			Any state transition.
> >>>>>   
> >>>>> +What:		/sys/bus/counter/devices/counterX/events_queue_size
> >>>>> +KernelVersion:	5.13
> >>>>> +Contact:	linux-iio@xxxxxxxxxxxxxxx
> >>>>> +Description:
> >>>>> +		Size of the Counter events queue in number of struct
> >>>>> +		counter_event data structures. The number of elements will be
> >>>>> +		rounded-up to a power of 2.
> >>>>> +
> >>>>>   What:		/sys/bus/counter/devices/counterX/name
> >>>>>   KernelVersion:	5.2
> >>>>>   Contact:	linux-iio@xxxxxxxxxxxxxxx
> >>>>> diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
> >>>>> index 16f02df7f73d..53eea894e13f 100644
> >>>>> --- a/drivers/counter/counter-chrdev.c
> >>>>> +++ b/drivers/counter/counter-chrdev.c
> >>>>> @@ -375,6 +375,29 @@ void counter_chrdev_remove(struct counter_device *const counter)
> >>>>>   	cdev_del(&counter->chrdev);
> >>>>>   }
> >>>>>   
> >>>>> +int counter_chrdev_realloc_queue(struct counter_device *const counter,
> >>>>> +				 size_t queue_size)
> >>>>> +{
> >>>>> +	int err;
> >>>>> +	DECLARE_KFIFO_PTR(events, struct counter_event);
> >>>>> +	unsigned long flags;
> >>>>> +
> >>>>> +	/* Allocate new events queue */
> >>>>> +	err = kfifo_alloc(&events, queue_size, GFP_ATOMIC);
> >>>>
> >>>> Is there any potential for losing events?
> >>>
> >>> We take the events_list_lock down below so we're safe against missing an
> >>> event, but past events currently unread in the queue will be lost.
> >>>
> >>> Shortening the size of the queue is inherently a destructive process if
> >>> we have more events in the current queue than can fit in the new queue.
> >>> Because we a liable to lose some events in such a case, I think it's
> >>> best to keep the behavior of this reallocation consistent and have it
> >>> provide a fresh empty queue every time, as opposed to sometimes dropping
> >>> events and sometimes not.
> >>>
> >>> I also suspect an actual user would be setting the size of their queue
> >>> to the required amount before they begin watching events, rather than
> >>> adjusting it sporadically during a live operation.
> >>>
> >>
> >> Absolutely agree.   As such I wonder if you are better off enforcing this
> >> behaviour?  If the cdev is open for reading, don't allow the fifo to be
> >> resized.
> >>
> >> Jonathan
> > 
> > I can't really think of a good reason not to, so let's enforce it: if
> > the cdev is open, then we'll return an EINVAL if the user attempts to
> > resize the queue.
> > 
> > What is a good way to check for this condition? Should I just call
> > kref_read() and see if it's greater than 1? For example, in
> > counter_chrdev_realloc_queue():
> > 
> > 	if (kref_read(&counter->dev.kobj.kref) > 1)
> > 		return -EINVAL;
> > 
> > William Breathitt Gray
> > 
> 
> Wouldn't EBUSY make more sense?

Yes, EBUSY would be better here because the operation isn't necessarily
invalid, just unavailable because someone else has the cdev open.

William Breathitt Gray

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