drm/msm: Fix a null pointer access in msm_gem_shrinker_count()

Adding an msm_gem_object object to the inactive_list before completing
its initialization is a bad idea because shrinker may pick it up from the
inactive_list. Fix this by making sure that the initialization is complete
before moving the msm_obj object to the inactive list.

This patch fixes the below error:
[10027.553044] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000068
[10027.573305] Mem abort info:
[10027.590160] ESR = 0x96000006
[10027.597905] EC = 0x25: DABT (current EL), IL = 32 bits
[10027.614430] SET = 0, FnV = 0
[10027.624427] EA = 0, S1PTW = 0
[10027.632722] Data abort info:
[10027.638039] ISV = 0, ISS = 0x00000006
[10027.647459] CM = 0, WnR = 0
[10027.654345] user pgtable: 4k pages, 39-bit VAs, pgdp=00000001e3a6a000
[10027.672681] [0000000000000068] pgd=0000000198c31003, pud=0000000198c31003, pmd=0000000000000000
[10027.693900] Internal error: Oops: 96000006 [#1] PREEMPT SMP
[10027.738261] CPU: 3 PID: 214 Comm: kswapd0 Tainted: G S 5.4.40 #1
[10027.745766] Hardware name: Qualcomm Technologies, Inc. SC7180 IDP (DT)
[10027.752472] pstate: 80c00009 (Nzcv daif +PAN +UAO)
[10027.757409] pc : mutex_is_locked+0x14/0x2c
[10027.761626] lr : msm_gem_shrinker_count+0x70/0xec
[10027.766454] sp : ffffffc011323ad0
[10027.769867] x29: ffffffc011323ad0 x28: ffffffe677e4b878
[10027.775324] x27: 0000000000000cc0 x26: 0000000000000000
[10027.780783] x25: ffffff817114a708 x24: 0000000000000008
[10027.786242] x23: ffffff8023ab7170 x22: 0000000000000001
[10027.791701] x21: ffffff817114a080 x20: 0000000000000119
[10027.797160] x19: 0000000000000068 x18: 00000000000003bc
[10027.802621] x17: 0000000004a34210 x16: 00000000000000c0
[10027.808083] x15: 0000000000000000 x14: 0000000000000000
[10027.813542] x13: ffffffe677e0a3c0 x12: 0000000000000000
[10027.819000] x11: 0000000000000000 x10: ffffff8174b94340
[10027.824461] x9 : 0000000000000000 x8 : 0000000000000000
[10027.829919] x7 : 00000000000001fc x6 : ffffffc011323c88
[10027.835373] x5 : 0000000000000001 x4 : ffffffc011323d80
[10027.840832] x3 : ffffffff0477b348 x2 : 0000000000000000
[10027.846290] x1 : ffffffc011323b68 x0 : 0000000000000068
[10027.851748] Call trace:
[10027.854264] mutex_is_locked+0x14/0x2c
[10027.858121] msm_gem_shrinker_count+0x70/0xec
[10027.862603] shrink_slab+0xc0/0x4b4
[10027.866187] shrink_node+0x4a8/0x818
[10027.869860] kswapd+0x624/0x890
[10027.873097] kthread+0x11c/0x12c
[10027.876424] ret_from_fork+0x10/0x18
[10027.880102] Code: f9000bf3 910003fd aa0003f3 d503201f (f9400268)
[10027.886362] ---[ end trace df5849a1a3543251 ]---
[10027.891518] Kernel panic - not syncing: Fatal exception

Signed-off-by: Akhil P Oommen <[e-mail]>
Signed-off-by: Rob Clark <[e-mail]>

drivers/gpu/drm/msm/msm_gem.c

···
 
 static int msm_gem_new_impl(struct drm_device *dev,
 		uint32_t size, uint32_t flags,
-		struct drm_gem_object **obj,
-		bool struct_mutex_locked)
+		struct drm_gem_object **obj)
 {
-	struct msm_drm_private *priv = dev->dev_private;
 	struct msm_gem_object *msm_obj;
 
 	switch (flags & MSM_BO_CACHE_MASK) {
···
 	INIT_LIST_HEAD(&msm_obj->submit_entry);
 	INIT_LIST_HEAD(&msm_obj->vmas);
 
-	if (struct_mutex_locked) {
-		WARN_ON(!mutex_is_locked(&dev->struct_mutex));
-		list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
-	} else {
-		mutex_lock(&dev->struct_mutex);
-		list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
-		mutex_unlock(&dev->struct_mutex);
-	}
-
 	*obj = &msm_obj->base;
 
 	return 0;
···
 		uint32_t size, uint32_t flags, bool struct_mutex_locked)
 {
 	struct msm_drm_private *priv = dev->dev_private;
+	struct msm_gem_object *msm_obj;
 	struct drm_gem_object *obj = NULL;
 	bool use_vram = false;
 	int ret;
···
 	if (size == 0)
 		return ERR_PTR(-EINVAL);
 
-	ret = msm_gem_new_impl(dev, size, flags, &obj, struct_mutex_locked);
+	ret = msm_gem_new_impl(dev, size, flags, &obj);
 	if (ret)
 		goto fail;
+
+	msm_obj = to_msm_bo(obj);
 
 	if (use_vram) {
 		struct msm_gem_vma *vma;
 		struct page **pages;
-		struct msm_gem_object *msm_obj = to_msm_bo(obj);
 
 		mutex_lock(&msm_obj->lock);
 
···
 		mapping_set_gfp_mask(obj->filp->f_mapping, GFP_HIGHUSER);
 	}
 
+	if (struct_mutex_locked) {
+		WARN_ON(!mutex_is_locked(&dev->struct_mutex));
+		list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
+	} else {
+		mutex_lock(&dev->struct_mutex);
+		list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
+		mutex_unlock(&dev->struct_mutex);
+	}
+
 	return obj;
 
 fail:
···
 struct drm_gem_object *msm_gem_import(struct drm_device *dev,
 		struct dma_buf *dmabuf, struct sg_table *sgt)
 {
+	struct msm_drm_private *priv = dev->dev_private;
 	struct msm_gem_object *msm_obj;
 	struct drm_gem_object *obj;
 	uint32_t size;
···
 
 	size = PAGE_ALIGN(dmabuf->size);
 
-	ret = msm_gem_new_impl(dev, size, MSM_BO_WC, &obj, false);
+	ret = msm_gem_new_impl(dev, size, MSM_BO_WC, &obj);
 	if (ret)
 		goto fail;
 
···
 	}
 
 	mutex_unlock(&msm_obj->lock);
+
+	mutex_lock(&dev->struct_mutex);
+	list_add_tail(&msm_obj->mm_list, &priv->inactive_list);
+	mutex_unlock(&dev->struct_mutex);
+
 	return obj;
 
 fail: