Re: [PATCH 2/3] drm/i915: close PM interrupt masking races in the rps work func

[Ben Widawsky <ben@bwidawsk.net>]

On Sun,  4 Sep 2011 17:35:01 +0200
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> This patch closes the following race:
> 
> We get a PM interrupt A, mask it, set dev_priv->iir = PM_A and kick
> of the work item. Scheduler isn't grumpy, so the work queue takes
> rps_lock, grabs pm_iir = dev_priv->pm_iir and pm_imr = READ(PMIMR).
> Note that pm_imr == pm_iir because we've just masked the interrupt
> we've got.
> 
> Now hw sends out PM interrupt B (not masked), we process it and mask
> it.  Later on the irq handler also clears PMIIR.
> 
> Then the work item proceeds and at the end clears PMIMR. Because
> (local) pm_imr == pm_iir we have
>         pm_imr & ~pm_iir == 0
> so all interrupts are enabled.
> 
> Hardware is still interrupt-happy, and sends out a new PM interrupt B.
> PMIMR doesn't mask B (it does not mask anything), PMIIR is cleared, so
> we get it and hit the WARN in the interrupt handler (because
> dev_priv->pm_iir == PM_B).
> 
> That's why I've moved the
>         WRITE(PMIMR, 0)
> up under the protection of the rps_lock. And write an uncoditional 0
> to PMIMR, because that's what we'll do anyway.
> 
> This races looks much more likely because we can arbitrarily extend
> the window by grabing dev->struct mutex right after the irq handler
> has processed the first PM_B interrupt.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/i915/i915_irq.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_irq.c
> b/drivers/gpu/drm/i915/i915_irq.c index 2fdd9f9..21ebcbd 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -383,6 +383,7 @@ static void gen6_pm_rps_work(struct work_struct
> *work) pm_iir = dev_priv->pm_iir;
>  	dev_priv->pm_iir = 0;
>  	pm_imr = I915_READ(GEN6_PMIMR);
> +	I915_WRITE(GEN6_PMIMR, 0);
>  	spin_unlock_irq(&dev_priv->rps_lock);
>  
>  	if (!pm_iir)
> @@ -420,7 +421,6 @@ static void gen6_pm_rps_work(struct work_struct
> *work)
>  	 * an *extremely* unlikely race with gen6_rps_enable() that
> is prevented
>  	 * by holding struct_mutex for the duration of the write.
>  	 */
> -	I915_WRITE(GEN6_PMIMR, pm_imr & ~pm_iir);
>  	mutex_unlock(&dev_priv->dev->struct_mutex);
>  }
>  

How about this:

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index 55518e3..3bc1479 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -415,12 +415,7 @@ static void gen6_pm_rps_work(struct work_struct *work)
        gen6_set_rps(dev_priv->dev, new_delay);
        dev_priv->cur_delay = new_delay;
 
-       /*
-        * rps_lock not held here because clearing is non-destructive. There is
-        * an *extremely* unlikely race with gen6_rps_enable() that is prevented
-        * by holding struct_mutex for the duration of the write.
-        */
-       I915_WRITE(GEN6_PMIMR, pm_imr & ~pm_iir);
+       I915_WRITE(GEN6_PMIMR, pm_imr & dev_priv->pm_iir);
        mutex_unlock(&dev_priv->dev->struct_mutex);
 }

Ben