Re: [PATCH v2] usb: roles: set switch registered flag early on

[Heikki Krogerus]

On Thu, Feb 06, 2025 at 11:39:50AM -0800, Elson Roy Serrao wrote:
> The role switch registration and set_role() can happen in parallel as they
> are invoked independent of each other. There is a possibility that a driver
> might spend significant amount of time in usb_role_switch_register() API
> due to the presence of time intensive operations like component_add()
> which operate under common mutex. This leads to a time window after
> allocating the switch and before setting the registered flag where the set
> role notifications are dropped. Below timeline summarizes this behavior
> 
> Thread1				|	Thread2
> usb_role_switch_register()	|
> 	|			|
> 	---> allocate switch	|
> 	|			|
> 	---> component_add()	|	usb_role_switch_set_role()
> 	|			|	|
> 	|			|	--> Drop role notifications
> 	|			|	    since sw->registered
> 	|			|	    flag is not set.
> 	|			|
> 	--->Set registered flag.|
> 
> To avoid this, set the registered flag early on in the switch register
> API.
> 
> Fixes: b787a3e78175 ("usb: roles: don't get/set_role() when usb_role_switch is unregistered")
> cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Elson Roy Serrao <quic_eserrao@xxxxxxxxxxx>

Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>

> ---
> Changes in v2:
>  - Set the switch registered flag from the get-go as suggested by
>    Heikki.
>  - Modified subject line and commit next as per the new logic.
>  - Link to v1: https://lore.kernel.org/all/20250127230715.6142-1-quic_eserrao@xxxxxxxxxxx/
> 
>  drivers/usb/roles/class.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/roles/class.c b/drivers/usb/roles/class.c
> index c58a12c147f4..30482d4cf826 100644
> --- a/drivers/usb/roles/class.c
> +++ b/drivers/usb/roles/class.c
> @@ -387,8 +387,11 @@ usb_role_switch_register(struct device *parent,
>  	dev_set_name(&sw->dev, "%s-role-switch",
>  		     desc->name ? desc->name : dev_name(parent));
>  
> +	sw->registered = true;
> +
>  	ret = device_register(&sw->dev);
>  	if (ret) {
> +		sw->registered = false;
>  		put_device(&sw->dev);
>  		return ERR_PTR(ret);
>  	}
> @@ -399,8 +402,6 @@ usb_role_switch_register(struct device *parent,
>  			dev_warn(&sw->dev, "failed to add component\n");
>  	}
>  
> -	sw->registered = true;
> -
>  	/* TODO: Symlinks for the host port and the device controller. */
>  
>  	return sw;
> -- 
> 2.17.1

-- 
heikki