Re: [PATCH] drm/i915: Move drm_framebuffer_unreference out of struct_mutex for takeover

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Mar 26, 2015 at 12:39:40PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> intel_user_framebuffer_destroy() requires the struct_mutex for its
> object bookkeeping, so this means that all calls to
> drm_framebuffer_unreference must be held without that lock.
> 
> This is a simplified version of the identically named patch by Chris Wilson.
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=89166
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index cb50854..0788507 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -14020,11 +14020,21 @@ void intel_modeset_gem_init(struct drm_device *dev)
>  					       c->primary->fb,
>  					       c->primary->state,
>  					       NULL)) {
> +			/*
> +			 * We must drop struct_mutex when calling
> +			 * drm_framebuffer_unreference and it is safe to do so
> +			 * because it is not needed at this point anyway.
> +			 * At this stage the driver is still single-threaded and
> +			 * we are taking it only to silence a warning in
> +			 * intel_pin_and_fence_fb_obj.
> +			 */
> +			mutex_unlock(&dev->struct_mutex);
>  			DRM_ERROR("failed to pin boot fb on pipe %d\n",
>  				  to_intel_crtc(c)->pipe);
>  			drm_framebuffer_unreference(c->primary->fb);
>  			c->primary->fb = NULL;
>  			update_state_fb(c->primary);
> +			mutex_lock(&dev->struct_mutex);
>  		}
>  	}
>  	mutex_unlock(&dev->struct_mutex);

Just grab the mutex around the pin_and_fence inside the loop. It doesn't
protect anything else.

-- 
Ville Syrjälä
Intel OTC
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