gve: Add adminq mutex lock

We were depending on the rtnl_lock to make sure there is only one adminq
command running at a time. But some commands may take too long to hold
the rtnl_lock, such as the upcoming flow steering operations. For such
situations, it can temporarily drop the rtnl_lock, and replace it for
these operations with a new adminq lock, which can ensure the adminq
command execution to be thread-safe.
Signed-off-by: Ziwei Xiao <ziweixiao@google.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240625001232.1476315-2-ziweixiao@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/google/gve/gve.h

@@ -724,6 +724,7 @@ struct gve_priv {
 	union gve_adminq_command *adminq;
 	dma_addr_t adminq_bus_addr;
 	struct dma_pool *adminq_pool;
+	struct mutex adminq_lock; /* Protects adminq command execution */
 	u32 adminq_mask; /* masks prod_cnt to adminq size */
 	u32 adminq_prod_cnt; /* free-running count of AQ cmds executed */
 	u32 adminq_cmd_fail; /* free-running count of AQ cmds failed */

drivers/net/ethernet/google/gve/gve_adminq.c

@@ -284,6 +284,7 @@ int gve_adminq_alloc(struct device *dev, struct gve_priv *priv)
 			    &priv->reg_bar0->adminq_base_address_lo);
 		iowrite32be(GVE_DRIVER_STATUS_RUN_MASK, &priv->reg_bar0->driver_status);
 	}
+	mutex_init(&priv->adminq_lock);
 	gve_set_admin_queue_ok(priv);
 	return 0;
 }
@@ -511,28 +512,29 @@ static int gve_adminq_issue_cmd(struct gve_priv *priv,
 	return 0;
 }
 
-/* This function is not threadsafe - the caller is responsible for any
- * necessary locks.
- * The caller is also responsible for making sure there are no commands
- * waiting to be executed.
- */
 static int gve_adminq_execute_cmd(struct gve_priv *priv,
 				  union gve_adminq_command *cmd_orig)
 {
 	u32 tail, head;
 	int err;
 
+	mutex_lock(&priv->adminq_lock);
 	tail = ioread32be(&priv->reg_bar0->adminq_event_counter);
 	head = priv->adminq_prod_cnt;
-	if (tail != head)
-		// This is not a valid path
-		return -EINVAL;
+	if (tail != head) {
+		err = -EINVAL;
+		goto out;
+	}
 
 	err = gve_adminq_issue_cmd(priv, cmd_orig);
 	if (err)
-		return err;
+		goto out;
 
-	return gve_adminq_kick_and_wait(priv);
+	err = gve_adminq_kick_and_wait(priv);
+
+out:
+	mutex_unlock(&priv->adminq_lock);
+	return err;
 }
 
 /* The device specifies that the management vector can either be the first irq