[Intel-gfx] Deadlock in intel_user_framebuffer_destroy()

[Chris Wilson]

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 at 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.

t a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
index 6372cfc..a9b8c43 100644
--- a/drivers/gpu/drm/i915/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/intel_fbdev.c
@@ -209,8 +209,6 @@ static int intelfb_create(struct drm_fb_helper *helper,
        int size, ret;
        bool prealloc = false;
 
-       mutex_lock(&dev->struct_mutex);
-
        if (intel_fb &&
            (sizes->fb_width > intel_fb->base.width ||
             sizes->fb_height > intel_fb->base.height)) {
@@ -221,6 +219,9 @@ static int intelfb_create(struct drm_fb_helper *helper,
                drm_framebuffer_unreference(&intel_fb->base);
                intel_fb = ifbdev->fb = NULL;
        }
+
+       mutex_lock(&dev->struct_mutex);
+
        if (!intel_fb || WARN_ON(!intel_fb->obj)) {
                DRM_DEBUG_KMS("no BIOS fb, allocating a new one\n");
                ret = intelfb_alloc(helper, sizes);

-- 
Chris Wilson, Intel Open Source Technology Centre