Re: [PATCH net-next v7 3/5] net: devmem: implement autorelease token management

On Wed, Nov 19, 2025 at 07:37:10PM -0800, Bobby Eshleman wrote:

...

@@ -292,25 +327,67 @@ net_devmem_bind_dmabuf(struct net_device *dev,

...

• /* Enforce system-wide autorelease mode consistency for RX bindings.

• * TX bindings don't use autorelease (always false) since tokens aren't
  

• * leaked in TX path. Only RX bindings must all have the same
  

• * autorelease mode, never mixed.
  

• *
  

• * We use the xarray's lock to atomically check xa_empty() and toggle
  

• * the static key, avoiding the race where two new bindings may try to
  

• * set the static key to different states.
  

• */
  

• xa_lock(&net_devmem_dmabuf_bindings);
• if (direction == DMA_FROM_DEVICE) {

• if (!xa_empty(&net_devmem_dmabuf_bindings)) {
  

• 	bool mode;
  

• 	mode = static_key_enabled(&tcp_devmem_ar_key);
  

• 	/* When bindings exist, enforce that the mode does not
  

• 	 * change.
  

• 	 */
  

• 	if (mode != autorelease) {
  

• 		NL_SET_ERR_MSG_FMT(extack,
  

• 				   "System already configured with autorelease=%d",
  

• 				   mode);
  

• 		err = -EINVAL;
  

• 		goto err_unlock_xa;
  

• 	}
  

• } else {
  

• 	/* First binding sets the mode for all subsequent
  

• 	 * bindings.
  

• 	 */
  

• 	if (autorelease)
  

• 		static_branch_enable(&tcp_devmem_ar_key);
  

• 	else
  

• 		static_branch_disable(&tcp_devmem_ar_key);
  

Hi Bobby,

This code runs inside xa_lock, which is a spinlock.

But static_branch_enable() and static_branch_disable() may sleep due to some combination of taking a mutex and cpu_read_lock.

Flagged by Claude Code with https://github.com/masoncl/review-prompts/

• }
  

• }
• err = __xa_alloc_cyclic(&net_devmem_dmabuf_bindings, &binding->id,

• 		binding, xa_limit_32b, &id_alloc_next,
  

• 		GFP_KERNEL);
  

  if (err < 0)

• goto err_free_chunks;
  

• goto err_unlock_xa;
  

• xa_unlock(&net_devmem_dmabuf_bindings);

list_add(&binding->list, &priv->bindings); return binding; +err_unlock_xa:

• xa_unlock(&net_devmem_dmabuf_bindings);

...