[PATCH] drm/i915/gvt: Convert from atomic_t to refcount_t on intel_vgpu_ppgtt_spt->refcount

[Xiyu Yang]

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations

Signed-off-by: Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>
Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>
---
 drivers/gpu/drm/i915/gvt/gtt.c | 11 ++++++-----
 drivers/gpu/drm/i915/gvt/gtt.h |  3 ++-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/gtt.c b/drivers/gpu/drm/i915/gvt/gtt.c
index cc2c05e18206..62f3daff5a36 100644
--- a/drivers/gpu/drm/i915/gvt/gtt.c
+++ b/drivers/gpu/drm/i915/gvt/gtt.c
@@ -841,7 +841,7 @@ static struct intel_vgpu_ppgtt_spt *ppgtt_alloc_spt(
 	}
 
 	spt->vgpu = vgpu;
-	atomic_set(&spt->refcount, 1);
+	refcount_set(&spt->refcount, 1);
 	INIT_LIST_HEAD(&spt->post_shadow_list);
 
 	/*
@@ -927,18 +927,19 @@ static struct intel_vgpu_ppgtt_spt *ppgtt_alloc_spt_gfn(
 
 static inline void ppgtt_get_spt(struct intel_vgpu_ppgtt_spt *spt)
 {
-	int v = atomic_read(&spt->refcount);
+	int v = refcount_read(&spt->refcount);
 
 	trace_spt_refcount(spt->vgpu->id, "inc", spt, v, (v + 1));
-	atomic_inc(&spt->refcount);
+	refcount_inc(&spt->refcount);
 }
 
 static inline int ppgtt_put_spt(struct intel_vgpu_ppgtt_spt *spt)
 {
-	int v = atomic_read(&spt->refcount);
+	int v = refcount_read(&spt->refcount);
 
 	trace_spt_refcount(spt->vgpu->id, "dec", spt, v, (v - 1));
-	return atomic_dec_return(&spt->refcount);
+	refcount_dec(&spt->refcount);
+	return refcount_read(&spt->refcount);
 }
 
 static int ppgtt_invalidate_spt(struct intel_vgpu_ppgtt_spt *spt);
diff --git a/drivers/gpu/drm/i915/gvt/gtt.h b/drivers/gpu/drm/i915/gvt/gtt.h
index 3bf45672ef98..944c2d0739df 100644
--- a/drivers/gpu/drm/i915/gvt/gtt.h
+++ b/drivers/gpu/drm/i915/gvt/gtt.h
@@ -38,6 +38,7 @@
 #include <linux/kref.h>
 #include <linux/mutex.h>
 #include <linux/radix-tree.h>
+#include <linux/refcount.h>
 
 #include "gt/intel_gtt.h"
 
@@ -243,7 +244,7 @@ struct intel_vgpu_oos_page {
 
 /* Represent a vgpu shadow page table. */
 struct intel_vgpu_ppgtt_spt {
-	atomic_t refcount;
+	refcount_t refcount;
 	struct intel_vgpu *vgpu;
 
 	struct {
-- 
2.7.4