Patch "drm/vmwgfx: Fix a deadlock in dma buf fence polling" has been added to the 6.6-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

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

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     drm-vmwgfx-fix-a-deadlock-in-dma-buf-fence-polling.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


From e58337100721f3cc0c7424a18730e4f39844934f Mon Sep 17 00:00:00 2001
From: Zack Rusin <zack.rusin@broadcom.com>
Date: Mon, 22 Jul 2024 14:41:13 -0400
Subject: drm/vmwgfx: Fix a deadlock in dma buf fence polling

From: Zack Rusin <zack.rusin@broadcom.com>

commit e58337100721f3cc0c7424a18730e4f39844934f upstream.

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: 2298e804e96e ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_fence.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

--- a/drivers/gpu/drm/vmwgfx/vmwgfx_fence.c
+++ b/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
 {
 	struct vmw_fence_obj *fence =
 		container_of(f, struct vmw_fence_obj, base);
-
 	struct vmw_fence_manager *fman = fman_from_fence(fence);
 
-	spin_lock(&fman->lock);
-	list_del_init(&fence->head);
-	--fman->num_fence_objects;
-	spin_unlock(&fman->lock);
+	if (!list_empty(&fence->head)) {
+		spin_lock(&fman->lock);
+		list_del_init(&fence->head);
+		spin_unlock(&fman->lock);
+	}
 	fence->destroy(fence);
 }
 
@@ -257,7 +256,6 @@ static const struct dma_fence_ops vmw_fe
 	.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
 		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(st
 				      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(st
 		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,


Patches currently in stable-queue which might be from zack.rusin@broadcom.com are

queue-6.6/drm-vmwgfx-trigger-a-modeset-when-the-screen-moves.patch
queue-6.6/drm-vmwgfx-fix-a-deadlock-in-dma-buf-fence-polling.patch
queue-6.6/drm-vmwgfx-fix-overlay-when-using-screen-targets.patch