Re: [PATCH 3/3] drm/i915: close rps work vs. rps disable races

[Ben Widawsky <ben@bwidawsk.net>]

On Sun, Sep 04, 2011 at 05:35:02PM +0200, Daniel Vetter wrote:
> The rps disabling code wasn't properly cancelling outstanding work
> items. Also add a comment that explains why we're not racing with
> the work item, that could again unmask interrupts.
> 
> This also fixes a bug on module unload because nothing was properly
> syncing up with that work item, possibly leading to it being run after
> freeing dev_priv or removing the module code.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/i915/intel_display.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 56a8554..ccd4600 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -7483,10 +7483,16 @@ void gen6_disable_rps(struct drm_device *dev)
>  
>  	I915_WRITE(GEN6_RPNSWREQ, 1 << 31);
>  	I915_WRITE(GEN6_PMINTRMSK, 0xffffffff);
> -	I915_WRITE(GEN6_PMIER, 0);
> +	/* Complete PM interrupt masking here doesn't race with the rps work
> +	 * item again unmasking PM interrupts because that is using PMIMR
> +	 * (logically sitting atop of PMINTRMSK) to mask interrupts. */
> +	cancel_work_sync(&dev_priv->rps_work);
>  
> +	/* Clear PMIMR and dev_priv->pm_iir in case outstanding work gets
> +	 * cancelled before having run. */

This comment is wrong, you are clearing PMIER, not IMR. IMR gets cleared
in enable() iirc

>  	spin_lock_irq(&dev_priv->rps_lock);
>  	dev_priv->pm_iir = 0;
> +	I915_WRITE(GEN6_PMIER, 0);
>  	spin_unlock_irq(&dev_priv->rps_lock);
>  
>  	I915_WRITE(GEN6_PMIIR, I915_READ(GEN6_PMIIR));
> -- 
> 1.7.6
> 

I'm not sure this actually fixes a problem. The existing code:
1. disables all interrupts (no more can occur).
2. sets pm_iir to 0 safe in rps lock
<workqueues can run at this point, but IMR has no effect with IER = 0>

I think you should do the cancel work sync somewhere in the code before
module unload (to be correct). I just don't think this fixes a race.

Ben