drm/vmwgfx: Fix a deadlock in dma buf fence polling

Introduce a version of the fence ops that on release doesn't remove
the fence from the pending list, and thus doesn't require a lock to
fix poll->fence wait->fence unref deadlocks.

vmwgfx overwrites the wait callback to iterate over the list of all
fences and update their status, to do that it holds a lock to prevent
the list modifcations from other threads. The fence destroy callback
both deletes the fence and removes it from the list of pending
fences, for which it holds a lock.

dma buf polling cb unrefs a fence after it's been signaled: so the poll
calls the wait, which signals the fences, which are being destroyed.
The destruction tries to acquire the lock on the pending fences list
which it can never get because it's held by the wait from which it
was called.

Old bug, but not a lot of userspace apps were using dma-buf polling
interfaces. Fix those, in particular this fixes KDE stalls/deadlock.
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Fixes: 2298e804 ("drm/vmwgfx: rework to new fence interface, v2")
Cc: Broadcom internal kernel review list <bcm-kernel-feedback-list@broadcom.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.2+
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Reviewed-by: Martin Krastev <martin.krastev@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240722184313.181318-2-zack.rusin@broadcom.com

drivers/gpu/drm/vmwgfx/vmwgfx_fence.c

@@ -32,7 +32,6 @@
 #define VMW_FENCE_WRAP (1 << 31)
 
 struct vmw_fence_manager {
-	int num_fence_objects;
 	struct vmw_private *dev_priv;
 	spinlock_t lock;
 	struct list_head fence_list;
@@ -124,13 +123,13 @@ static void vmw_fence_obj_destroy(struct dma_fence *f)
 {
 	struct vmw_fence_obj *fence =
 		container_of(f, struct vmw_fence_obj, base);
-
 	struct vmw_fence_manager *fman = fman_from_fence(fence);
 
+	if (!list_empty(&fence->head)) {
 		spin_lock(&fman->lock);
 		list_del_init(&fence->head);
-	--fman->num_fence_objects;
 		spin_unlock(&fman->lock);
+	}
 	fence->destroy(fence);
 }
 
@@ -257,7 +256,6 @@ static const struct dma_fence_ops vmw_fence_ops = {
 	.release = vmw_fence_obj_destroy,
 };
 
-
 /*
  * Execute signal actions on fences recently signaled.
  * This is done from a workqueue so we don't have to execute
@@ -355,7 +353,6 @@ static int vmw_fence_obj_init(struct vmw_fence_manager *fman,
 		goto out_unlock;
 	}
 	list_add_tail(&fence->head, &fman->fence_list);
-	++fman->num_fence_objects;
 
 out_unlock:
 	spin_unlock(&fman->lock);
@@ -403,7 +400,7 @@ static bool vmw_fence_goal_new_locked(struct vmw_fence_manager *fman,
 				      u32 passed_seqno)
 {
 	u32 goal_seqno;
-	struct vmw_fence_obj *fence;
+	struct vmw_fence_obj *fence, *next_fence;
 
 	if (likely(!fman->seqno_valid))
 		return false;
@@ -413,7 +410,7 @@ static bool vmw_fence_goal_new_locked(struct vmw_fence_manager *fman,
 		return false;
 
 	fman->seqno_valid = false;
-	list_for_each_entry(fence, &fman->fence_list, head) {
+	list_for_each_entry_safe(fence, next_fence, &fman->fence_list, head) {
 		if (!list_empty(&fence->seq_passed_actions)) {
 			fman->seqno_valid = true;
 			vmw_fence_goal_write(fman->dev_priv,