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

From: William Breathitt Gray <[email-masked]>
Date: Sun, 28 Feb 2021 11:46:46 +0900

On Sat, Feb 27, 2021 at 03:18:47PM +0000, Jonathan Cameron wrote:
> On Fri, 26 Feb 2021 09:03:48 +0900
> William Breathitt Gray <[email-masked]> 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 <[email-masked]> 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 <[email-masked]> 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 <[email-masked]>
> > > > > > Signed-off-by: William Breathitt Gray <[email-masked]>
> > > > > > ---
> > > > > >  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:	[email-masked]
> > > > > > +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:	[email-masked]
> > > > > > 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;
> In theory at least you might want the kobj.kref to be incremented
> for other reasons than just open.   So to keep different concepts
> separate, perhaps it's worth a separate variable somewhere to
> track whether the file is open currently.
> 
> However, it's reasonable (I think) to assume the kref will have a
> minimum value if open, so perhaps what you suggest works fine.
> 
> Jonathan

I noticed an open() operation could occur right after this check, so
we'll need a mutex here to ensure the the queue size is not modified
during use. Because of that, I'll create a separate variable to track
this and use a mutex_trylock() instead of the kref to test whether to
return -EBUSY.

William Breathitt Gray
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