On 08/02/2016 03:12 AM, Brian Norris wrote:

Hi all,

I'm seeing the following warnings when I read from an IIO char device,
with CONFIG_DEBUG_ATOMIC_SLEEP=y. I'm testing a v4.4 kernel, but AFAICT,
nothing too relevant has changed between that and v4.7:

[...]

Have any of you seen this kind of issue before (perhaps most IIO users
are not using CONFIG_DEBUG_ATOMIC_SLEEP)? If the WARNING is really
correct, then this problem has really been around a while. It looks like
we have a wait_event_interruptible() called, with this call chain in the
'condition' path:

  iio_buffer_ready()
    -> iio_buffer_data_available() (i.e., iio_kfifo_buf_data_available())
      -> mutex_lock()

Calling mutex_lock() means we clobber the TASK_INTERRUPTIBLE state with
TASK_RUNNING -- hence, the WARNING. Should this be using a spinlock
instead? Or is there some way to refactor this to avoid calling these
sleeping functions in the wait_event*() condition?

Hi,

Yes, this is an issue, thanks for pointing this out. It has been there for a
while, my fault, sorry for that. We need a solution like pointed out in this
article (https://lwn.net/Articles/628628/).

- Lars