drm/vmwgfx: Prevent unmapping active read buffers

The kms paths keep a persistent map active to read and compare the cursor
buffer. These maps can race with each other in simple scenario where:
a) buffer "a" mapped for update
b) buffer "a" mapped for compare
c) do the compare
d) unmap "a" for compare
e) update the cursor
f) unmap "a" for update
At step "e" the buffer has been unmapped and the read contents is bogus.

Prevent unmapping of active read buffers by simply keeping a count of
how many paths have currently active maps and unmap only when the count
reaches 0.

Fixes: 485d98d472d5 ("drm/vmwgfx: Add support for CursorMob and CursorBypass 4")
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.19+
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240816183332.31961-2-zack.rusin@broadcom.com
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
index f42ebc4a7c22552ef0f518390bd725201ea6c34e..a0e433fbcba67cab54eb3e2cf52eff3ffe18ac3c 100644 (file)
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
@@ -360,6 +360,8 @@ void *vmw_bo_map_and_cache_size(struct vmw_bo *vbo, size_t size)
        void *virtual;
        int ret;
 
+       atomic_inc(&vbo->map_count);
+
        virtual = ttm_kmap_obj_virtual(&vbo->map, &not_used);
        if (virtual)
                return virtual;
@@ -383,11 +385,17 @@ void *vmw_bo_map_and_cache_size(struct vmw_bo *vbo, size_t size)
  */
 void vmw_bo_unmap(struct vmw_bo *vbo)
 {
+       int map_count;
+
        if (vbo->map.bo == NULL)
                return;
 
-       ttm_bo_kunmap(&vbo->map);
-       vbo->map.bo = NULL;
+       map_count = atomic_dec_return(&vbo->map_count);
+
+       if (!map_count) {
+               ttm_bo_kunmap(&vbo->map);
+               vbo->map.bo = NULL;
+       }
 }
 
 
@@ -421,6 +429,7 @@ static int vmw_bo_init(struct vmw_private *dev_priv,
        vmw_bo->tbo.priority = 3;
        vmw_bo->res_tree = RB_ROOT;
        xa_init(&vmw_bo->detached_resources);
+       atomic_set(&vmw_bo->map_count, 0);
 
        params->size = ALIGN(params->size, PAGE_SIZE);
        drm_gem_private_object_init(vdev, &vmw_bo->tbo.base, params->size);

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
index 62b4342d5f7c5053aaca65487e2607c8ea6949a1..43b5439ec9f7600753a2d58c52414cf873d521cb 100644 (file)
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.h
@@ -71,6 +71,8 @@ struct vmw_bo_params {
  * @map: Kmap object for semi-persistent mappings
  * @res_tree: RB tree of resources using this buffer object as a backing MOB
  * @res_prios: Eviction priority counts for attached resources
+ * @map_count: The number of currently active maps. Will differ from the
+ * cpu_writers because it includes kernel maps.
  * @cpu_writers: Number of synccpu write grabs. Protected by reservation when
  * increased. May be decreased without reservation.
  * @dx_query_ctx: DX context if this buffer object is used as a DX query MOB
@@ -90,6 +92,7 @@ struct vmw_bo {
        u32 res_prios[TTM_MAX_BO_PRIORITY];
        struct xarray detached_resources;
 
+       atomic_t map_count;
        atomic_t cpu_writers;
        /* Not ref-counted.  Protected by binding_mutex */
        struct vmw_resource *dx_query_ctx;