drm/nvc0/fb: take lock in nvc0_ram_put()

Kernel panic caused by list corruption in ltcg seems to indicate a
concurrency issue.

Take mutex of pfb like nv50_ram_put() to eliminate concurrency.

V2: Separate critical section into separate function, avoid taking the
    lock twice on NVC0
Signed-off-by: Roy Spliet <r.spliet@student.tudelft.nl>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/subdev/fb/priv.h

@@ -81,7 +81,7 @@ void nv44_fb_tile_prog(struct nouveau_fb *, int, struct nouveau_fb_tile *);
 void nv46_fb_tile_init(struct nouveau_fb *, int i, u32 addr, u32 size,
 		       u32 pitch, u32 flags, struct nouveau_fb_tile *);
 
-void nv50_ram_put(struct nouveau_fb *, struct nouveau_mem **);
+void __nv50_ram_put(struct nouveau_fb *, struct nouveau_mem *);
 extern int nv50_fb_memtype[0x80];
 
 #endif

drivers/gpu/drm/nouveau/core/subdev/fb/ramnv50.c

@@ -27,17 +27,10 @@
 #include "priv.h"
 
 void
-nv50_ram_put(struct nouveau_fb *pfb, struct nouveau_mem **pmem)
+__nv50_ram_put(struct nouveau_fb *pfb, struct nouveau_mem *mem)
 {
 	struct nouveau_mm_node *this;
-	struct nouveau_mem *mem;
 
-	mem = *pmem;
-	*pmem = NULL;
-	if (unlikely(mem == NULL))
-		return;
-
-	mutex_lock(&pfb->base.mutex);
 	while (!list_empty(&mem->regions)) {
 		this = list_first_entry(&mem->regions, typeof(*this), rl_entry);
 
@@ -46,6 +39,19 @@ nv50_ram_put(struct nouveau_fb *pfb, struct nouveau_mem **pmem)
 	}
 
 	nouveau_mm_free(&pfb->tags, &mem->tag);
+}
+
+void
+nv50_ram_put(struct nouveau_fb *pfb, struct nouveau_mem **pmem)
+{
+	struct nouveau_mem *mem = *pmem;
+
+	*pmem = NULL;
+	if (unlikely(mem == NULL))
+		return;
+
+	mutex_lock(&pfb->base.mutex);
+	__nv50_ram_put(pfb, mem);
 	mutex_unlock(&pfb->base.mutex);
 
 	kfree(mem);

drivers/gpu/drm/nouveau/core/subdev/fb/ramnvc0.c

@@ -33,11 +33,19 @@ void
 nvc0_ram_put(struct nouveau_fb *pfb, struct nouveau_mem **pmem)
 {
 	struct nouveau_ltcg *ltcg = nouveau_ltcg(pfb);
+	struct nouveau_mem *mem = *pmem;
 
-	if ((*pmem)->tag)
-		ltcg->tags_free(ltcg, &(*pmem)->tag);
+	*pmem = NULL;
+	if (unlikely(mem == NULL))
+		return;
 
-	nv50_ram_put(pfb, pmem);
+	mutex_lock(&pfb->base.mutex);
+	if (mem->tag)
+		ltcg->tags_free(ltcg, &mem->tag);
+	__nv50_ram_put(pfb, mem);
+	mutex_unlock(&pfb->base.mutex);
+
+	kfree(mem);
 }
 
 int