linux-kernel - Re: [PATCH] usb: roles: cache usb roles received during switch registration

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Message-ID: <c3fca109-b9ad-40c8-8f4a-033c48a86850@quicinc.com>
Date: Wed, 5 Feb 2025 15:15:43 -0800
From: Elson Serrao <quic_eserrao@...cinc.com>
To: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
CC: <gregkh@...uxfoundation.org>, <xu.yang_2@....com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] usb: roles: cache usb roles received during switch
 registration



On 2/3/2025 6:40 AM, Heikki Krogerus wrote:
> On Mon, Jan 27, 2025 at 03:07:15PM -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, cache the last role received and set it once the switch
>> registration is complete. Since we are now caching the roles based on
>> registered flag, protect this flag with the switch mutex.
> 
> Instead, why not just mark the switch registered from the get-go?
> 
> diff --git a/drivers/usb/roles/class.c b/drivers/usb/roles/class.c
> index c58a12c147f4..cf38be82d397 100644
> --- a/drivers/usb/roles/class.c
> +++ b/drivers/usb/roles/class.c
> @@ -387,6 +387,8 @@ 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) {
>                 put_device(&sw->dev);
> @@ -399,8 +401,6 @@ 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;
> 

Thank you for the feedback.

Yes that works as well. Wasn't entirely sure if we can set that flag from the get-go before calling device_register().
But guess we can reset the flag if device_register fails since that is the only failure path in role_switch_register().

I will upload v2 with this modification.

Best Regards,
Elson