[Intel-gfx] [PATCH] drm/i915: Fix failure paths around initial fbdev allocation

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

We had three failure modes here:

1.
Deadlock in intelfb_alloc failure path where it calls
drm_framebuffer_remove, which grabs the struct mutex and intelfb_create
(caller of intelfb_alloc) was already holding it.

2.
Double unreference on the object if __intel_framebuffer_create fails
since both it and the caller (intelfb_alloc) do the unreference.

3.
Deadlock in intelfb_create failure path where it calls
drm_framebuffer_unreference, which grabs the struct mutex and
intelfb_create was already holding it.

v2:
   * Reformat commit msg to 72 chars. (Lukas Wunner)
   * Added third failure mode. (Lukas Wunner)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Fixes: 60a5ca015ffd2aacfe5674b5a401cd2a37159e07
Reported-By: Lukas Wunner <lukas at wunner.de>
Tested-By: Lukas Wunner <lukas at wunner.de>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Lukas Wunner <lukas at wunner.de>
---
Ville, you suggested some changes earlier;

"""
I suggest removing the unref from __intel_framebuffer_create().
"""

Do you view that as an improvement in code clarity/organisation,
or you think my version is actually wrong for some reason?
---
 drivers/gpu/drm/i915/intel_fbdev.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
index 2a1724e..118420b 100644
--- a/drivers/gpu/drm/i915/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/intel_fbdev.c
@@ -141,7 +141,7 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
 {
 	struct intel_fbdev *ifbdev =
 		container_of(helper, struct intel_fbdev, helper);
-	struct drm_framebuffer *fb;
+	struct drm_framebuffer *fb = NULL;
 	struct drm_device *dev = helper->dev;
 	struct drm_mode_fb_cmd2 mode_cmd = {};
 	struct drm_i915_gem_object *obj;
@@ -159,6 +159,8 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
 	mode_cmd.pixel_format = drm_mode_legacy_fb_format(sizes->surface_bpp,
 							  sizes->surface_depth);
 
+	mutex_lock(&dev->struct_mutex);
+
 	size = mode_cmd.pitches[0] * mode_cmd.height;
 	size = PAGE_ALIGN(size);
 	obj = i915_gem_object_create_stolen(dev, size);
@@ -172,8 +174,9 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
 
 	fb = __intel_framebuffer_create(dev, &mode_cmd, obj);
 	if (IS_ERR(fb)) {
+		/* Drops object reference on failure. */
 		ret = PTR_ERR(fb);
-		goto out_unref;
+		goto out;
 	}
 
 	/* Flush everything out, we'll be doing GTT only from now on */
@@ -183,15 +186,18 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
 		goto out_fb;
 	}
 
+	mutex_unlock(&dev->struct_mutex);
+
 	ifbdev->fb = to_intel_framebuffer(fb);
 
 	return 0;
 
 out_fb:
-	drm_framebuffer_remove(fb);
-out_unref:
 	drm_gem_object_unreference(&obj->base);
 out:
+	mutex_unlock(&dev->struct_mutex);
+	if (fb)
+		drm_framebuffer_remove(fb);
 	return ret;
 }
 
@@ -209,8 +215,6 @@ 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)) {
@@ -225,7 +229,7 @@ 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");
@@ -237,6 +241,8 @@ static int intelfb_create(struct drm_fb_helper *helper,
 	obj = intel_fb->obj;
 	size = obj->base.size;
 
+	mutex_lock(&dev->struct_mutex);
+
 	info = framebuffer_alloc(0, &dev->pdev->dev);
 	if (!info) {
 		ret = -ENOMEM;
@@ -307,7 +313,6 @@ static int intelfb_create(struct drm_fb_helper *helper,
 out_unpin:
 	i915_gem_object_ggtt_unpin(obj);
 	drm_gem_object_unreference(&obj->base);
-out_unlock:
 	mutex_unlock(&dev->struct_mutex);
 	return ret;
 }
-- 
2.4.2