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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/26/2015 01:30 PM, Ville Syrjälä wrote:
> 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.

Well the comment says so, but this way it only grabs and releases it 
once if there are multiple active crtcs and nothing fails. So I was 
hoping the comment was enough to explain the reality, even though the 
other option would be more obvious code strictly speaking.

Regards,

Tvrtko
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