Re: [PATCH v12 15/17] counter: Implement events_queue_size sysfs attribute

[David Lechner]

On 7/10/21 11:08 AM, David Lechner wrote:
> On 7/10/21 10:43 AM, David Lechner wrote:
> > On 7/10/21 5:25 AM, William Breathitt Gray wrote:
> > > > and after that
> > > >
> > > > [   16.564403] ================================================
> > > > [   16.570725] WARNING: lock held when returning to user space!
> > > > [   16.577044] 5.13.0-next-20210706+ #4 Not tainted
> > > > [   16.582198] ------------------------------------------------
> > > > [   16.588507] cat/331 is leaving the kernel with locks still held!
> > > > [   16.595214] 1 lock held by cat/331:
> > > > [   16.599103]  #0: ffff888102bb3630
> > > > (&counter->chrdev_lock){+.+.}-{3:3}, at: counter_chrdev_open+0x21/0x60
> > > > [counter]
> > > >
> > > > Jarkko
> > > I'm not sure how to resolve this warning. The purpose of this lock is to
> > > limit chrdev to a single open at a time. To accomplish this I grab this
> > > lock in counter_chrdev_open() and hold it until counter_chrdev_release()
> > > is called. Is there a better way to accomplish this?
> >
> > How about using an atomic flag, e.g test_and_set_bit()?
>
> Another option could be to rethink it at a higher level and avoid the
> need for a lock (and sysfs attribute) altogether. For example, would it
> work to (re)allocate the kfifo buffer in the COUNTER_ENABLE_EVENTS_IOCTL
> callback and add a parameter to that ioctl to specify the buffer size
> (with units of events rather than bytes)?

Thinking about it a bit more, this probably isn't a good suggestion.
It would create issues with being able to start reading from the chrdev
before enabling events.