Re: [Intel-gfx] [PATCH 2/3] drm/i915: Fix incorrect assert about pending power domain async-put work

[Anshuman Gupta <anshuman.gupta@intel.com>]

On 2021-05-26 at 20:07:28 +0530, Imre Deak wrote:
> It's possible that an already dequeued put_async_work() will release the
> reference (*) that was put asynchronously after the dequeue happened.
> This leaves an async-put work pending, without any reference to release.
> A subsequent async-put may trigger the
> 
> drm_WARN_ON(!queue_delayed_work(&power_domains->async_put_work));
> 
> warn due to async_put_work() still pending. To avoid the warn, cancel
> the pending async_put_work() when releasing the reference at (*) above.
Not able to visulize the race here between __intel_display_power_put_async
and intel_display_power_put_async_work() considering both are protected by
power_domains->lock.
queue_delayed_work_on() documentation says following about return value.
"Return: %false if @work was already on a queue, %true otherwise."
AFAIU from the doc, queue_delayed_work will return false only when
work was in queue after dequeued put_async_work() it should return true ?
Thanks,
Anshuman Gupta.
> 
> Fixes: https://gitlab.freedesktop.org/drm/intel/-/issues/3421
> Fixes: https://gitlab.freedesktop.org/drm/intel/-/issues/2289
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display_power.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_display_power.c b/drivers/gpu/drm/i915/display/intel_display_power.c
> index 2f7d1664c4738..a95bbf23e6b7c 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_power.c
> +++ b/drivers/gpu/drm/i915/display/intel_display_power.c
> @@ -2263,6 +2263,12 @@ intel_display_power_put_async_work(struct work_struct *work)
>  			fetch_and_zero(&power_domains->async_put_domains[1]);
>  		queue_async_put_domains_work(power_domains,
>  					     fetch_and_zero(&new_work_wakeref));
> +	} else {
> +		/*
> +		 * Cancel the work that got queued after this one got dequeued,
> +		 * since here we released the corresponding async-put reference.
> +		 */
> +		cancel_delayed_work(&power_domains->async_put_work);
>  	}
>  
>  out_verify:
> -- 
> 2.27.0
> 
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