[Intel-gfx] [PATCH 08/12] drm/i915: Fix struct mutex vs. RPS lock inversion

[Chris Wilson]

On Tue, Feb 02, 2016 at 11:06:26AM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> RPS lock must be taken before the struct_mutex to avoid
> locking inversion. So stop grabbing it for the whole
> powersave initialization and instead only take it during
> the sections which need it.
> 
> Also, struct_mutex is not needed any more since dedicated
> RPS lock was added in:
> 
>    commit 4fc688ce79772496503d22263d61b071a8fb596e
>    Author: Jesse Barnes <jbarnes at virtuousgeek.org>
>    Date:   Fri Nov 2 11:14:01 2012 -0700
> 
>        drm/i915: protect RPS/RC6 related accesses (including PCU) with a new mutex
> 
> Based on prototype patch by Chris Wilson and a subsequent
> mailing list discussion involving Ville, Imre, Chris and
> Daniel.
> 
> v2: More details in the commit.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Imre Deak <imre.deak at intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 4 ----
>  drivers/gpu/drm/i915/intel_pm.c      | 9 +++++----
>  2 files changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 5018295cd92b..af0d33a3697a 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -15995,9 +15995,7 @@ void intel_modeset_gem_init(struct drm_device *dev)
>  	struct drm_i915_gem_object *obj;
>  	int ret;
>  
> -	mutex_lock(&dev->struct_mutex);
>  	intel_init_gt_powersave(dev);
> -	mutex_unlock(&dev->struct_mutex);
>  
>  	intel_modeset_init_hw(dev);
>  
> @@ -16077,9 +16075,7 @@ void intel_modeset_cleanup(struct drm_device *dev)
>  
>  	intel_cleanup_overlay(dev);
>  
> -	mutex_lock(&dev->struct_mutex);
>  	intel_cleanup_gt_powersave(dev);
> -	mutex_unlock(&dev->struct_mutex);

The whitespace no longer conveys meaning, it used to be to clearly mark
the mutex section.

> @@ -5235,6 +5233,7 @@ static void valleyview_setup_pctx(struct drm_device *dev)
>  out:
>  	DRM_DEBUG_DRIVER("PCBR: 0x%08x\n", I915_READ(VLV_PCBR));
>  	dev_priv->vlv_pctx = pctx;
> +	mutex_unlock(&dev->struct_mutex);
>  }
>  
>  static void valleyview_cleanup_pctx(struct drm_device *dev)
> @@ -5244,8 +5243,10 @@ static void valleyview_cleanup_pctx(struct drm_device *dev)
>  	if (WARN_ON(!dev_priv->vlv_pctx))
>  		return;
>  
> +	mutex_lock(&dev->struct_mutex);
>  	drm_gem_object_unreference(&dev_priv->vlv_pctx->base);
>  	dev_priv->vlv_pctx = NULL;
> +	mutex_unlock(&dev->struct_mutex);

This made me smile.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre