Re: [PATCH/RFC RESEND] leds: Use set_brightness_work for brightness_set ops that can sleep

Pavel Machek
2015-07-01T07:43:40Z

On Wed 2015-07-01 09:28:52, Jacek Anaszewski wrote:
> On 06/30/2015 07:46 PM, Pavel Machek wrote:
> >On Tue 2015-06-30 15:06:19, Jacek Anaszewski wrote:
> >>On 06/30/2015 01:58 PM, Pavel Machek wrote:
> >>>On Tue 2015-06-30 10:01:08, Jacek Anaszewski wrote:
> >>>>This patch rearranges the core LED subsystem code, so that it
> >>>>now removes from drivers the responsibility of using work queues
> >>>>internally in case their brightness_set ops can sleep.
> >>>>Addition of two flags: LED_BRIGHTNESS_FAST and LED_BLINK_DISABLE
> >>>>as well as new_brightness_value property to the struct led_classdev
> >>>>allows for employing existing set_brightness_work to do the job.
> >>>>The modifications allow also to get rid of brightness_set_sync op,
> >>>>as flash LED devices can now be handled properly only basing on the
> >>>>SET_BRIGHTNESS_SYNC flag.
> >>>
> >>>Are you sure this is good idea?
> >>>
> >>>You'll now use single callback for blocking and non-blocking
> >>>behaviour. I'm pretty sure stuff like lockdep will have some fun with
> >>>that.
> >>
> >>I enabled "Lock Debugging" options and didn't get any warning.
> >>Could you describe the use case you are thinking of?
> >
> >You may get one when one of the sleeping functions uses some lock...
>
> Drivers which use spin_lock in their brightness_set op will have to
> set LED_BRIGHTNESS_FAST flag, which will instruct the LED core to
> call the op synchronously. On the other hand drivers which can sleep
> in their brightness_set op won't set the flag, which will make LED core
> delegating the op to the work queue task. It is also possible that
> driver with brightness_set op that can sleep set SET_BRIGHTNESS_SYNC
> flag - then LED core will call it in a synchronous way from
> led_brightness_set and it will schedule work queue task in case
> the op is called from triggers.

I understand this "works".

> If you want to NAK the patch, please come up with detailed analysis
> on how it can cause problems. Without this I infer that you didn't
> spend a second on analyzing the code. This is counterproductive.

NAK.

Because calling two functions with different semantics through same
function pointer is extremely ugly, and _will_ cause lockdep
problems. Talk to the lockdep people for details.

Pavel
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