drm/i915: Fix failure paths around initial fbdev allocation

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

[Daniel Vetter on why struct_mutex needs to be locked in the second half
of intelfb_create: "The vma [for the fbdev] is pinned, the problem is
that we re-lookup it a few times, which is racy. We should instead track
the vma directly, but oh well we don't."]

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

v5:
   * Rebase on drm-intel-nightly 2015y-09m-01d-09h-06m-08s UTC,
     rephrase commit message. (Jani Nicula)

v6:
   * In intelfb_alloc, if __intel_framebuffer_create failed,
     fb will be an ERR_PTR, thus not null. So in the failure
     path we need to check for IS_ERR_OR_NULL to avoid calling
     drm_framebuffer_remove on the ERR_PTR. (Lukas Wunner)
   * Since this is init code a drm_framebuffer_unreference should
     be all we need. drm_framebuffer_remove is for framebuffers
     that userspace has created - and is getting somewhat
     defeatured. (Daniel Vetter)

v7:
   * Clarify why struct_mutex needs to be locked in the second half
     of intelfb_create. (Daniel Vetter)

Fixes: 60a5ca015ffd ("drm/i915: Add locking around
    framebuffer_references--")
Reported-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
[Lukas: Create v3 + v4 + v5 + v6 + v7 based on Tvrtko's v2]
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/47d4e88c91b3bf0f7a280cabec54c8c8cf0cf6f2.1446892879.git.lukas@wunner.de
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
index 2240e709ede6330f2c213865774162b68440e1dc..db82ad45de13c8d358337841cb249cf5af214d62 100644 (file)
--- a/drivers/gpu/drm/i915/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/intel_fbdev.c
@@ -119,7 +119,7 @@ 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_i915_private *dev_priv = to_i915(dev);
        struct drm_mode_fb_cmd2 mode_cmd = {};
@@ -138,6 +138,8 @@ 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);
 
@@ -165,16 +167,19 @@ static int intelfb_alloc(struct drm_fb_helper *helper,
        ret = intel_pin_and_fence_fb_obj(NULL, fb, NULL);
        if (ret) {
                DRM_ERROR("failed to pin obj: %d\n", ret);
-               goto out_fb;
+               goto out;
        }
 
+       mutex_unlock(&dev->struct_mutex);
+
        ifbdev->fb = to_intel_framebuffer(fb);
 
        return 0;
 
-out_fb:
-       drm_framebuffer_remove(fb);
 out:
+       mutex_unlock(&dev->struct_mutex);
+       if (!IS_ERR_OR_NULL(fb))
+               drm_framebuffer_unreference(fb);
        return ret;
 }
 
@@ -192,8 +197,6 @@ 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)) {
@@ -208,7 +211,7 @@ 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");
@@ -220,6 +223,8 @@ static int intelfb_create(struct drm_fb_helper *helper,
        obj = intel_fb->obj;
        size = obj->base.size;
 
+       mutex_lock(&dev->struct_mutex);
+
        info = drm_fb_helper_alloc_fbi(helper);
        if (IS_ERR(info)) {
                ret = PTR_ERR(info);
@@ -281,7 +286,6 @@ out_destroy_fbi:
 out_unpin:
        i915_gem_object_ggtt_unpin(obj);
        drm_gem_object_unreference(&obj->base);
-out_unlock:
        mutex_unlock(&dev->struct_mutex);
        return ret;
 }