Re: [PATCH v8 20/22] counter: Implement events_queue_size sysfs attribute
From: William Breathitt Gray
Date: Thu Feb 25 2021 - 19:04:52 EST
On Sun, Feb 21, 2021 at 03:51:40PM +0000, Jonathan Cameron wrote:
>
On 2021年2月18日 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 2021年2月12日 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
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> > > to a power of 2 due to a requirement of the kfifo_alloc function called
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> > > 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 ++
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> > > 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
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> > > index 847e96f19d19..f6cb2a8b08a7 100644
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> > > --- a/Documentation/ABI/testing/sysfs-bus-counter
>
> > > +++ b/Documentation/ABI/testing/sysfs-bus-counter
>
> > > @@ -212,6 +212,14 @@ Description:
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> > > both edges:
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> > > Any state transition.
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> > >
>
> > > +What: /sys/bus/counter/devices/counterX/events_queue_size
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> > > +KernelVersion: 5.13
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> > > +Contact: linux-iio@xxxxxxxxxxxxxxx
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> > > +Description:
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> > > + 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
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> > > KernelVersion: 5.2
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> > > Contact: linux-iio@xxxxxxxxxxxxxxx
>
> > > diff --git a/drivers/counter/counter-chrdev.c b/drivers/counter/counter-chrdev.c
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> > > index 16f02df7f73d..53eea894e13f 100644
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> > > --- 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)
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> > > cdev_del(&counter->chrdev);
>
> > > }
>
> > >
>
> > > +int counter_chrdev_realloc_queue(struct counter_device *const counter,
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> > > + size_t queue_size)
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> > > +{
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> > > + int err;
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> > > + DECLARE_KFIFO_PTR(events, struct counter_event);
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> > > + unsigned long flags;
>
> > > +
>
> > > + /* Allocate new events queue */
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> > > + 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
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> 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
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> 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
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> adjusting it sporadically during a live operation.
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>
>
>
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
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