Re: Deadlock in intel_user_framebuffer_destroy()

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

Hi,

On 06/15/2015 08:53 AM, Chris Wilson wrote:
> On Mon, Jun 15, 2015 at 09:44:15AM +0300, Jani Nikula wrote:
>> On Wed, 03 Jun 2015, Chris Wilson <chris@chris-wilson.co.uk> wrote:
>>> On Wed, Jun 03, 2015 at 03:43:32PM +0200, Lukas Wunner wrote:
>>>> Hi,
>>>>
>>>> a deadlock was introduced by commit 60a5ca015ffd2aacfe5674b5a401cd2a37159e07
>>>>
>>>> Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>> Date:   Fri Jun 13 11:10:53 2014 +0300
>>>>
>>>>      drm/i915: Add locking around framebuffer_references--
>>>>
>>>>
>>>> The commit amended intel_display.c:intel_user_framebuffer_destroy() with
>>>> mutex_lock(&dev->struct_mutex).
>>>>
>>>> A few weeks prior Chris Wilson had amended intel_fbdev.c:intelfb_create()
>>>> with a call to drm_framebuffer_unreference() while &dev->struct_mutex is
>>>> locked (commit edd586fe705e819bc711b5ed7194a0b6f9f1a7e1, "drm/i915: Discard
>>>> BIOS framebuffers too small to accommodate chosen mode").
>>>>
>>>
>>> Just move the mutex_lock down a step.
>>
>> Lucas, did you try this?
>
> There's a goto unlock that also needed to be disabled, such as
>
> diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
> index dda99c0d6be1..fc7ec5138fb7 100644
> --- a/drivers/gpu/drm/i915/intel_fbdev.c
> +++ b/drivers/gpu/drm/i915/intel_fbdev.c
> @@ -213,8 +213,6 @@ static int intelfb_create(struct drm_fb_helper *helper,
>          bool prealloc = false;
>          int ret;
>
> -       mutex_lock(&dev->struct_mutex);
> -
>          if (intel_fb &&
>              (sizes->fb_width > intel_fb->base.width ||
>               sizes->fb_height > intel_fb->base.height)) {
> @@ -229,7 +227,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
>                  DRM_DEBUG_KMS("no BIOS fb, allocating a new one\n");
>                  ret = intelfb_alloc(helper, sizes);
>                  if (ret)
> -                       goto out_unlock;
> +                       return ret;
>                  intel_fb = ifbdev->fb;
>          } else {
>                  DRM_DEBUG_KMS("re-using BIOS fb\n");
> @@ -241,6 +239,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
>          obj = intel_fb->obj;
>          vma = i915_gem_obj_to_ggtt(obj, NULL);
>
> +       mutex_lock(&dev->struct_mutex);
>          info = framebuffer_alloc(0, &dev->pdev->dev);
>          if (!info) {
>                  ret = -ENOMEM;
> @@ -311,7 +310,6 @@ static int intelfb_create(struct drm_fb_helper *helper,
>   out_unpin:
>          drm_gem_object_unreference(&obj->base);
> -out_unlock:
>          mutex_unlock(&dev->struct_mutex);
>          return ret;
>   }
>

intelfb_alloc wants struct_mutex, both for __intel_framebuffer_create 
and pin_and_fence. And also there is that double obj unreference in the 
failure path from the former Jani spotted.

Regards,

Tvrtko
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